AI am making a cake for a friend's wedding and she needs a three tier cake, she wants the top tier real cake and the bottom two tiers as a dummy. I've never worked with cake dummies so my first question is one, will the dummies hold the weight of a top tier even if it's say a four or six inch top tier and also my second question is can someone recommend a good website to order from? I want affordable dummies with good quality. TIA :)

First question, yes, the dummies will be strong enough to hold a cake of that size on top of them. No dowels needed.  Second answer, go to Dallas Foam and you can order.  They do so many things for their customers.

AOk awesome thanks :)

Cake Grandma is right! Foam is stronger then you think and will be able to provide a great base for your top tier. Keep in mind that you can also reuse your dummies for future cakes, if the bride will let you have them back   Ive gotten my dummies from buyfoamblocks.com if you need another option for purchasing your dummies!  Good luck!

AShould I use one center dowel to avoid it from sliding off?

That may be smart just to ensure that nothing goes wrong. You can also use piping gel if you don't want to stick a dowel through the cake. If the room temp. is average then you might not need it but if it is outside I am afraid the heat may make the icing melt and slide so a dowel could be a big help!