My favourite topic! I could look at fabric for quilting all day. Try:
Favourite Fabrics on Ebay - great 100% cotton fabrics for around £7 a metre. (Sorry I can't link, Ebay is banned at my work! But you should come across them fairly easily when browsing cotton fabric for quilting/craft.
Emma's Fabric Studio - www.emmasfabricstudio.co.uk/ - just a delight to browse
Dragonfly Fabrics - www.dragonflyfabrics.co.uk/ - one of the few places who will send you samples for a small charge.
www.myfabrics.co.uk/tilda-fabrics.html - haven't tried this company but I have my eye on them for being one of the cheapest (by a fraction) places to buy lovely Tilda fabric.
Fabric Rehab - fabricrehab.co.uk/ - for loads of fun and playful designs.
Sometimes for colour matching, feel etc there's nothing better than going to a local fabric shop.
I disagree about wanting something that stretches - if you're doing anything like squares, triangles, star patters etc you want something that doesn't stretch easily so that you can line up your seams properly.
Also when ironing fabric or pressing seams it's important to just press, don't drag the iron over as that will stretch the fabric and make your seams out of whack. Really depends on your project though, I wouldn't worry about this if doing crazy patchwork for example.