I made my own spacers using some 18mm MDF, just use your original speakers as a guide and draw around them and then cut out the centre hole depending on the size of the new replacement speakers. But if there are some fitting spacers on the market that will fit then you could easily just buy some of these
I'm running 17cm JBL's in the fronts on mine and 13cm Pioneers in the rears, JBL's cost me about £45(normal cost £70ish) and the Pioneers were around £30(normal £40ish)