Autotrader is your best bet, eBay is full of scammers and time wasters
Remember that changing the car for a diesel (i'm assuming thats what you are doing as you mention cost) - Takes along time to re-coup the outlay of cash.
I LPG'd my 2.2 Omega, cost around £700. So imagine if instead you spent £700+ on getting a replacement diesel.
LPG basically saves you around 40% + in fuel cost, going from say 30mpg average to an
effective 50mpg+
It still takes around 8 to 10,000 miles or so to get that cost back, before you start to save money.
So again assuming you do 40 miles a day, every day, 5 days a week, every month for 12 months, £700 spend won't save you any money. Only after about 13/14th month will you save.
Something to bear in mind