Keep an eye out on ebay. You'll need to get validated before you can buy or sell on here, which is a security measure, so it's not a good idea to try. Do you prefer the feel of a BMX or a mountainbike? That will often give you the answer to whether you want a mod or a stock, 26" bike. Mod IS short, for modified, as in modified bmx. Originally, mod bikes were custom jobs where people modified bmx bikes to make them better for trials. Stock, on the other hand... back when trials was a young sport, people found that montainbikes were good for trials as they came, so they could be ridden stock, standard.