Tough system to find games for. I got my games a few years back, before the systems became so much more common on ebay. I was paying about $50-$75 a piece for boxed games at the time, regardless of source. But at the time, there were just a few of us trying to find them so it was a lot easier and nobody pushing the prices up.
I had quite a few offers for just the 3 games that I traded to BlueBMW, up to about $85 a piece for loose copies. But he had some X68000 goodness that I just couldn't resist so they all went to him along with my second system and controller.
Truthfully, there isn't a whole lot worth playing once you get past CUG, Boomzoo, and Speedy Dragon. Sango Fighter is fun, but I used to play it a lot back in the day on PC so I think it's mostly nostalgia for me. The rest are cool from the standpoint of being legit Taiwanese releases, but for gameplay they aren't worth grabbing.
Feel free to bounce any questions you have off of me. Unfortunately, I can't really point you towards any reliable sources for the games.