Look in the classifieds. None for sale on this site.
Do you want a tent or a bivvy?
Having tried both I can tell you that charging out of a tent in the pitch black when you can only crawl out is not easy.
Go for a bivvy and look in the angling mags and large tackle shops for end of line bivvies.
Kirkgate anglers in Leeds (0113 243 4880 have a one man cypry dome for ?70 if you can stretch that far.
Some larger shops will do you a post dated cheque system where you give them 3 or 6 post dated cheques, each for 1/3 or 1/6 of what you owe. Its cheap short term finance and interest free. Might be worth a shot.