I've got some puzzle mats just off my actual mat area. The ones I've got are definitely thick enough to roll on, but wouldn't cut it for take downs unless you doubled up on them, and still I'm not sure that I'd want to get put down hard on them.
So, if you're looking at just practicing ground work, check out softtiles.com
I'm in no way affiliated, but I couldn't find any place who had mats that were less expensive.
The mats Cageside listed would definitely give more cushion, but if you're stictly starting on the ground and on a really tight budget check out Soft Tiles. They shipped really quickly, and I covered a 10x7ft space just off my mats for just over 100 bucks I think.