Yamaha DTXpress/DTXplorer/DTXtreme

I just got my DTXpress III a few months ago and am loving it so far,
but i am finding myself wanting more pads.. especially cymbals. I see
all these photos of nice cymbals and pad additions, wheres a good
place to buy from that will sell the pads individually? Also, do pads
made from other makes of drums such as Roland for instance mix fine
with the Yamaha DTXpress III? Thanks ahead of time!