It reads 5 count for the mesh on there, personally I would feel the holes could be a little too large and the bettas may be able to bite eachother through that. I use 7 count which is just big enough to let plenty of water flow through.
I searched craft shops but only found it available through wool shops or anywhere that does cross-stitch and needlecraft. I got it for 90 pence a sheet. I believe Hobbycraft stock it too and I also found it in Beatties. The report binders I got from a cheap stationery shop for 99p for 10 and you need 4 of them, so a lot cheaper than buying a purpose made divider for £8.99!
Good luck in finding some.