[olivine]

Try b&q or some DIY/home wear shop. You might be able to get cut offs of Lino for cheap. Slate is brilliant but you MUST have your mat linked to a thermostat for slate. Slate conducts heat extremely well and can burn your reptile if not monitored, I'm speaking from experience as my Leo wouldn't go anywhere neat her slate when i didn't have a stat, I then touched the slate and it was to hot for me to even touch. So yea thermostat always!!

Thermostats are absolutely essential irrespective of the substrate being used (indeed, they should be used for all artificial heat sources).

OP: I use slate roof tiles for my leo vivs, which I nabbed for free when a relative was having their house reroofed. It might be worth contacting local roofing companies to see you if can blag any surplus / broken tiles (for the latter, just make sure you smooth off any sharp / rough edges before using them).