Buying a new mat depends on a few things, including where you're at with your practice. If you're just starting out, even a camping roll mat will give you the padding and comfort to get you started. As you progress, you may well want something with a bit more grip though. Some of you may also be concerned to get a mat that is environmentally sound too.
have good selections. You can buy mats on the highstreet in places like Argos, TK Max and in some supermarkets, but be careful that cheap mats aren't slippy or stretch easily. Cheaper mats also tend not to offer the environemental benefits. The annual yoga show in London can be a good place to get a good deal on a mat, and plenty of choice.
Things to look for:
I've used mats by www.yogamatters.com and they wear well and are reasonably grippy - good general practice mats at good value.
Bolsters - you can improvise with a robust blanket rolled up and slipped into a pillow case
Blocks - improvise with an old phone directory or old text book
Straps - improvise with a tie, dressing gown belt or scarf