Thanks to those who replied.
Costco isn't really an option, but the link to Opus was interesting. I hadn't realised that Olmec was freely available again after it's past difficulties in distribution.
After some searching, the best bet looks to be using Olmec gloss at 22p/A4 sheet. It's cheap enough not to fret about when using for testing, but still good enough for decent prints.
Silverprint sell it at a better price than Opus and they're a company I'm happy to support with my money.