I looked into this a little while ago and the cheapest way of getting them is from sweeden and to buy 150 works out at 1.60 each which sounds ok but 150 is 320 quid I was sniffing about for people to chip in and get say 50 each but got no interest.
Also you need bottles with screw of crown caps and they're not easy to find in the uk.
I've since stopped bottling beer and just keg everything