Truvativ Firex chainset replacement middle 32 tooth chainring & bolts
According to this bike shop it takes Shimano 4 bolt 104mm/74mm BCD chainrings, so you should be able to use the Shimano rings. If you have a spare bike and the right tools, take the cranks off, remove the ring and take both into a shop. That way you/they can check the fit before you buy a chainring. It's one of those times when you're better off paying a premium for LBS service so you don't end up buying multiple wrong parts online.

I don't know how (un)common those chainrings are, but we still get them as a new stock item in Australia and there seem to be lots of hits in google