Laying in a stock of wood sounds like a great idea but in my opinion there are only a few sorts that are good to always have in stock and you can never cover every type and size that a plan for a model will recommend.
Basics like 1/16th and 1/8th plywood, either Miralyte or Lite ply are used a lot. You pay more and more every time a supplier cuts sheet wood down into smaller and smaller strips. All hardwoods like Balsa, Mahogany and so on are horrendously expensive to start with.
It is far better, if you can, to have a circular saw and cut wood down as you need it and so I only stock basic Balsa sheet in 1/16th, 1/8th and 1/4 inch sizes and cut them into strips as needed.
The comment about Skips is a true one and the ability to convert whatever good quality timber into usable sizes on a saw becomes very good value. I rarely, if ever buy hardwood, but I use them all the time, all from what people throw away.
Pine is perfect for making strip if you buy the 'CLS' or 'Russian Ends' of 3x2 from Wickes or other builders yards, but only select lengths that have a light, tight even grain pattern and no knots. It is very good value and can be machined into strip easily as needed. All my chine strips and spray strips are made from these types of pine which respond well to soaking or steaming and bends easily.
Always look out for things like slats from garden furniture (Teak), old table legs, newel posts or desks (often Beech), Replaced window frames and conservatories (Mahogany), We don't often find a use for Oak but if you find a piece (Church refurbs) then keep it just in case. Learn about woods so you can recognise them and their uses.
You would be far better equipped by spending £100 on a hobby type table saw than a pile of strip wood. Anything special you need you can buy as and when.
I also use SLEC as mentioned above, I have also found Balsa Cabin very good for Plywood, Balsa, softwood strip, Spruce and so on, Foam and some plastics... I tend to order plastic sheet in larger sizes from Direct Plastics, Tower Plastics and also SHG marine. all very good. Then I use the saw to cut up plastic sheet into strip as well.
You could pay for the saw in a couple of models on what you save...and this is real scratch-building!
Just my slant on the subject.
Ron.