I can usually find cotton for 99p to £1.99 on eBay.
Also if you are wanting cheap jersey, keeping an eye out in Primark for jersey bedsheets is good. They don't always have them, but when they do they're perfect to stock up on.
I also have a roll of curtain lining I got very cheap (or maybe by mistake - they must've sent me more than the 13m I ordered considering I used 13m and still have a massive roll...)