I guess for specific things it depends where you're located.
But tips for general stuff for example: most Ikea stores have a discount/returned goods section & I've got a lot of very usable surfaces like desk/table tops & sections of kitchen counter tops for peanuts.
Stone merchants/tile stores etc always give away samples mostly for free & I've got metre square slabs of slate, granite, marble, etc - all perfectly useable for close in table top stuff.
Also got a perspex display maker locally who's happy to sell me off cuts for very little & his off cuts are a very good size due to the scale he works on.