From a builders merchant or timber merchant, you'll pay about £3/metre
for 8 by 2 floor joists - ask for regularised and treated (the usual
spec) - or even say you're using them for gardening and see if they
have anything they can't sell for construction due to warping or
splitting etc.

Alternatively ask any builders you see demolishing a domestic property
- they'll probably let you have the joists for little or nothing - but
you'll probably have to de-nail them.