Parts-N-More is not bad for new/repro parts. Some models more than other though.
They show a page worth of stuff for the DT250, but they also don't cross reference very well. So if you know what other models use the same part you're looking for, you can look them up as well.
Also try HVC Cycle and Economy Cycle. They focus more on the street 2strokes, but again, if you know what to cross reference...