The two basic sources for lumber are to mill your own (very fun, control over the way you like your wood cut, extremely slow to cut and to wait for it to season, and even if you get the trees for free, a far more expensive way to get wood in your shop) or to head on down to your local lumber yard and let go of your credit card.
Depending on where you live and the size of the city, you may have several choices. The biggest yards are generally the cheapest, but I rarely use them for a number of reasons. One is that they mostly do not want to sell to smaller shops buying just a few hundred feet and forget about trying to get a stick or two of lumber for a project. They also generally do not display their wood but just stack it up in huge banded bundles- several hundred board feet of wood. and will give you a dirty look if you ask them to drop it down so you can sort through. Never, but never go into one of them and ask for 1” or 2” lumber. They will whip around and go back to playing solitaire on their computers what idiot does not know that sawn hardwood lumber is referred to in quarters of an inch, in these examples 4/4 and 8/4. My two biggest issues with the big yards, however, are that they very rarely carry more than a handful of “popular “ species such as oak, walnut, maple, and cherry. And most of them will not let you sort through the lumber to find what suits you best. That, for me, is the deal breaker, as I consider lumber selection to be one of the most enjoyable and critical aspects of my work.
At the other end of the food-pyramid-of-wood are the local boutique lumber yards. Many are attached to tool stores, and they often put price stickers on each board. Red flag! This is the most expensive way to buy wood. And while they often have a dizzying array of species, and they encourage you to comb through them, I have yet to see one that has much of any one species at the same time, and that leaves the woodworker to have very little choice in the exact boards they may require for a project.
Above you will see an example of the best price, but little access or variety accompanied by surly salesmen, or too much selection, high prices, and not always the best quality.
In between lies perhaps the best of all worlds for small to medium-sized shops. One could even call them the “tweener” lumberyard. Prices better than the boutiques, not always terribly more than the big yards. Much more access to and ability to select exactly what you want. Prices probably not marked on each individual board, but maybe a board footage tally so you can see how much you are buying. And they will talk to you whether you refer to inches or quarters, although you may get a better price if you appear to know more about the business of buying lumber.