I'm a kiwi with a Sinar P kit that I've accumulated over the last few years via Trademe, Pro Gear in Parnell (AK) and my own home built bits

I started with an 8x10, then got a 4x5, found some Norma and F bits in a friends car boot (!) then realizing I had enough rail, bellows and bits and bobs built a 11x14" portrait only film holder for it (using the 8x10 rear standard).

I think having been around the NZ shops and trademe quite a bit you're not going to find an 8x10" and especially not a 5x7" conversion kit... Likelihood is that you'll find an 8x10" complete kit and buy that and then pilfer your 4x5 for parts. Eventually ending up with crazy cam like these guys:

16x20" !!!

Best of Luck