Best place to buy a bag of compost?

I used home base grow bags for most of last year seedlings. doesn't last well though for long term plants and most of its been added into the compost heap now. also no good for house plant compost goes white and stale very quickly

Best place to buy is wherever they are doing a 2 for 1 offer.  Or cheap broken bags.
So I will wander around B&Q, garden centres, Wilcos etc  looking for any good deals.
Of course what you buy is a totally different question.

Downside is that you can spend ten quid in petrol to save a fiver