My family minhag is to use tomatoes for karpas, what is the mekor for this and do any of you guys on miyodeya use tomatoes for karpas as well.
The karpas must have the bracha of ha'adamah, and indeed, over tomatoes you recite the bracha of borei pri ha'adamah. I have never used tomatoes myself, but I've heard of the custom. It's fine.
I use a boiled potato for the karpas; but it has to be boiled because raw potatoes are for one thing bad for your health but also they do not have the bracha of ha'adamah. Potatoes and onions are shehakol unless they've been cooked.