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. 

Yoga matters and The British Wheel of Yoga shop 

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:

• good grip - check it doesn't have a shiny surface, although some mats will improve their grip with use or a wash.
• no stretch - I once made the mistake of buying a thicker mat that I thought looked comfy and would give good padding and comfort, which it does, but then stretches horribly, making warrior poses and down dogs very hard work!
• scratch test - ideally the surface shouldn't scratch easily. Some of the cheaper mats are not very robust and scratching them with a nail will lift ridges and blobs of rubber very easily - these mats won't last long.

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