I bought them from Duramat:
https://duramat.co.uk/category/discount-packs/
Although they are 50cm sq they're shipped in sq mtr batches ie 4 tiles. I laid them like that but with hindsight I should have split them back into individual tiles since I had a couple that didn't want to interlock cleanly and the brute-force-and-ignorance approach damaged them (they did supply a hammer
). I've repaired the damage and it's not noticeable. They didn't suggest using a lubricant but after that I used some ArmorAll type product and it helped.