Cold frame advice

I Know Dobies sell cold frames but they are a bit pricey, you can buy cold frames from the Range, various styles and materials, there is a pvc covered frame for about £20. then polycarbonate about £50 then ofcourse glass which no doubt wil cost more for postage there is also Wilcos with and ebay, I don't remember there being any garden centers in Oban, but haven't been there for years