If dollars is your biggest concern, (and with the cost of some blanks, it should be!) I would recommend firewood. If you have access to a bandsaw then this is doubly easy. The majority of the wood I turn is stuff I've set aside while cutting fire wood. Either as I cut it into chunks or later when I split it in half to dry.
If you are an urban dweller, then keep your eyes open for places where they are cutting down trees. Often especially if it is the city they don't mind letting someone else take some, it's that much less they have to clean up and deal with. Also sometimes when they cut trees, it is stacked on the curb for later pick up, ask and you can probably get more than you can fit in your car.
Firewood gives you a lot of free wood to practice an learn on, and since it's free, you don't have to worry about 'messing it up'. I have yet to buy any wood for turning, but then I have had access to LOTS of firewood.
Another one might be if there is a mill nearby. You might be able to pick through and buy a bunch of slabs. I have my own mill, so provide my own slabs, but they can be very cheap, maybe even free, and you can cut circle banks from them. I use a lot of these as well, for flatter, thinner bowls plates. Often they are still quite pretty.