Have a gander at www.growell.co.uk
They sell cabinets that you can either buy readymade (for about £100), lined with mylar and have holes for intake/exhaust fans. They also sell kits that you can make your own with. If you cant afford that, then start looking at rubbish dumps and in skips on the side of the road - some people throw stuff out that can be bloody useful for a grow, and all it needs is a good clean.