Trying to source materials for free I think the main thing is to keep your eyes open and nab anything suitable you find as and when you see it, like when looking for pallets.
Basically you'll be eyeing any skip you drive past in case there's some wood or sheet material sticking out of it that you can take1, poking your nose into any house renovations or building sites to ask if they have anything they'll be throwing out2. Those are possibly the two most common ways to acquire 2x softwoods, ply and occasionally other materials like worktop offcuts for free 'on your travels'.
Another source worth definitely worth checking are the free-ads sites like Preloved and Gumtree, where stuff is offered for free or very cheaply basically to offload it and get it out of someone's hair as fast as possible (e.g. when clearing out a garage or a shed).
One last option worth trying is the local reclamation yard (or multiples if you have more than one within easy striking distance). These are of course not set up to allow members of the public to take stuff away but you lose nothing by asking politely of the staff. Explaining that you're only starting out and just want it to build stuff for the workshop/garage may gain you a sympathetic ear. This might not be viable depending on the staff (it's not uncommon to find them to be grumpy old farts to say the least..... dealing with the public all day every day will do that to a person!) or the council's H&S rules might be very tight and the blokes on site just can't allow it.
Last option I can think of is a semi-commercial route, going to a proper timber yard that stocks reasonable quality plywood and knows how to store it and asking if they have any plywood that's gotten edge damage or is too dirty to sell in the normal way3. This can, at the right yard, net you stuff for a steep discount.
1 Ask first before raiding a skip if at all possible, this is always best practice. And never trespass to get at one. Although when a skip is outside on public ground the stuff inside is generally considered fair game this is not always the case, and there are no legal precedents you can confidently use as a defence if the skip owner objects. In short, you might be guilty of theft if you don't ask and get permission.
2 This in particular can be surprisingly lucrative if you only need a finite amount of material, since a builder's discards from just one job could keep you in sheet goods for cabinets and jigs for the foreseeable future.
3 From being dropped or run into by the forklift, accidents happen even in the best-managed yards. The edge damage will need to be cut off, I wouldn't attempt to repair it as it's almost never worth the effort. As for the dirt, a surprising amount of surface filth can be cleaned off with a damp cloth and/or by scraping or sanding, and afterwards it can look practically new. I've even gotten reasonable-looking stuff out of builder's discards that were spattered with cement or mortar, it was quite a lot of work cleaning that off and the same won't always be doable but it gives you an idea of what you can deal with if necessary.