Many people do make their own boards. I buy mine. They're called drum boards and are usually about 1/2" thick and covered with silver, gold or white food-safe foil. They're usually available at most cake decorating supply stores. For large custom sized cakes, then definitely making the board is necessary.
They also sell them online if you don't have a store that sells them near you. (The closest store to me is 45 minutes away!!) I get mine from global sugar art. There are different sizes and shapes. They are cheaper in bulk.
I find them very worth it because it gives your cake a professional look.
You can also use foam core board from your local craft store (Hobby Lobby, Michael's, etc.) and cut them to the size you need. As a hobby baker, I never know what size boards I'll need for my next project, so I don't want to have to store a bunch of different sizes. I stock up on foam core board when on sale and have it ready when it's time to make a new board. This also allows me to let the recipient just throw it away when they are done with the cake - no worrying about getting an pricey board back!
You can also use foam core board from your local craft store (Hobby Lobby, Michael's, etc.) and cut them to the size you need. As a hobby baker, I never know what size boards I'll need for my next project, so I don't want to have to store a bunch of different sizes. I stock up on foam core board when on sale and have it ready when it's time to make a new board. This also allows me to let the recipient just throw it away when they are done with the cake - no worrying about getting an pricey board back!
I've heard of people using foam core but never done so myself. Do you buy it in a specific thickness or glue several of them together? Do you cover it in contact paper or food save foil afterwards? What are the chances they'll bend?
I like the drum boards for my standards size tiered cakes because I know they'll support the weight of the cake. Plus covering them in fondant is easy as well. And I'm not concerned about getting the board back because I've included the cost in my materials. But I've not found them overly pricey.
i've used foam core before and covered them with food safe foil afterwards. I've always glued 2 together for thickness and strength. I actually still prefer the premade cake-drums though. And yes, i also include them in my cost. I never ask for them back, but sometimes people offer and i always say "Yes!!" Then i can reuse them, and it goes towards profit.
I buy the 1/2" and haven't had an issue with bending. For my brother-in-law's wedding cake, I did glue two together because I was concerned about there being too much weight. DH thought one was fine, but I was thinking better be safe than sorry.
<-- was a single 1/2" foam core board covered with a black disposable table cloth (cake is on it's own board, taped to the main board).
You can cover them with cake foil or fondant or anything else you use. Ribbon can be glued, taped, or you can even use pins to secure it in place.
I don't take money for cakes or even supplies, so true cake drums are too pricey for me to have an array of sizes in stock - let alone, a place to store them all! Cake toys/tools have already taken over much of my home!! If I had a cake business and had the volume, I'm sure I'd prefer the premade boards. As with many other things in making cakes, it's best to find what works best for you.
by foam board you mean lthe stuff they have by the oak tag/posterboard? I never thought of that. I dont do A LOT of cakes but they are always different sizes so wood is probably not in budget. Oddly I live in a major area and we dont have ANY cake supply stores. Oh how I miss NY!
I use foamboard too. I use the 1/4" thickness and I glue them together to make it stronger, for heavy cakes. I use them as boards between the tiers also, covered with press and seal wrap. A couple times a year the craft stores will have foamcore on sale for a dollar each, and I stock up then.
I usually cover my base board with fondant. That way I can match the cake without having lots of colors of foil on hand. I don't really care for the silver foil look with every cake. It's easy to put fondant on foamcore, just roll it directly onto the foamcore and let the rolling pin go off the edges, and you'll end up with a nice rounded top edge. Then glue ribbon around the side. I usually use the 1/2" ribbon that hobby lobby has in the bridal section...it's about two dollars a roll and there's plenty on the roll for lots of cakes. In a pinch you can paint the ribbon with luster dust and vodka, after the glue holding it on the board has dried...if, for example, you need a blue ribbon and all you have is white.
Oh, to make cutting foamcore easier, make sure you have a nice sharp blade. I use the pocket knife style box cutter. When the edges start coming out a little jagged, it's time to put in a new blade.
I use them as boards between the tiers also, covered with press and seal wrap.
Do you have any issues with a centre wood dowel going through them when you use them between tiers?
Sharpen the end of the dowel with a pencil sharpener (that's never been used for pencils of course) and it goes through the foamcore like butter 
I use a variety of items. Foamcore, MDF, drums and even just 3 or 4 stacked standard cake boards. It all works.
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