Yah, sad isn't it. There are SO many species suitable for indoor bonsai cultivation, but only one or two are regularly offered in garden centers, malls, etc., and they're almost always unsuitable, or unsuitably prepared.
If you're starting with an older tree, you probably want to pick that out in person to be sure that there are no unsightly cuts, wire marks, etc. and that the design is acceptable to you. If you have more patience for growing your own (keeping in mind that it won't look like much for the first few years) there are a few sites that offer "starter plants". If you can't find suitable species in your area that are upto what you want, yes, you can get older trees on the internet. Ficus are very forgiving, so they'd be the older trees at which would want to start looking. There are a few sites that offer older Ficus, but, they're not necessarily going to be trees that are especially well care for and shaped, etc., especially if they're in an affordable price range. You can check out bonsaiboy.com and meehansminiatures.com for these trees.