Shelving material ?

[maz0]

Gumtree and Facebook-marketplace for left over stock from builders or the occasional wooden floorboard removal job. You can get some really good bargains on there. Decking and fence posts/slats are good treated wood as well.

 

[bigegg]

I use scaffolding board.
I pay £7 for a 13ft length - sometimes less.
I suppose it depends what you're using the shelving for.

for cheap off cuts of plywood, try a "forme maker", "ply liner" or "CNC routing" place