You don't say what sections you want, what quantities or for what
purpose (joinery/construction).

If you buy larger quantities from builders merchants, and you buy the
stuff they sell a lot of, and you keep a look out for specials,
savings can be made.

The high turnover lines are 18mm structural ply, 18mm osb, studding
and 50*200mm joists. None of them pretty materials.

If you're willing to consider reclaimed timber (and de-nail it
yourself) - there are a few yards dealing it, or just have the cheek
to walk up to smaller building works and ask if they have any scrap
materials.