I know many will hate these with a passion - fair enough - but this is the cheapest I've seen them to buy individually.
This isn't available online - just checked. GE 20W T3 LOW ENERGY STICK 10 YEAR ES CAP - they are 98p online.
Considering these used to be a few quid each not too long ago - this is so cheap!
The direct link will take you to the product online.