Where can I buy a plain good carrot cake? I want to decorate it myself so thats why I want it plain.
Most supermarkets have a baked goods section, you could try there. You could go to your local small bakery and ask if they could make you a plain one (usually small bakeries take orders for cakes etc). Or why don't you make one yourself!!
Unless you go to a really nice bakery you can probably bake a better carrot cake than you can buy. Like mandyloo said, you should have no problem ordering a carrot cake without decoration from any bakery.
Target actually has some pretty good carrot cake along with Meijer. Avoid walmart though as those are poorly made. Also consider go to a local bakery, they have the freshest and best stuff. Yummy .
You can order one from your local bakery and ask that it not be decorated. Avoid wal-mart, target, winn-dixie, and albertsons though - all of theirs get shipped in already made and decorated.