To practice regular buttercream, make a batch of the Wilton with all shortening, and use water to thin. You can re-use it as many times as you want. Royal icing is a difficult one-you can make a large batch and store it in an airtight jar, but I don't use it that often so I can't tell you how long it will last.
You can find practice boards online and have them laminated. For writing, you can print out some templates online (I just use MS Word). You can experiment with a lot of different writing styles that way. You can also pipe directly onto a piece of waxed paper with a template underneath.