Buying a lower and building yourself is an economical way to get what you want.
I agree that parts are hard to find, no doubt, but you can still get the job done, and for much cheaper than buying a complete rifle right now. Once you have the lower, you can pick up random parts as you find them. Gun shows are a great place to find cheap used parts like collapsable stocks, pins and springs, and flash hiders. The online prices are way higher for that type of stuff, but you can get a whole bag of stuff for next to nothing if you look for it.
And used parts are just as good as new ones in many situations. Stuff like a bolt carrier can cost you close to a hundred or more new, but can be found way cheaper elseware, and you won't lose any quality from a used one unless someone used it as a hammer.
To give you an idea, I just finished a build on a DPMS rifle in 308 last week that took me about a year to build. I bought the receiver last spring before prices went up. I didn't start buying major parts until after the election, and trying to buy the already scarce parts for a 308 type AR rifle is hard enough during the good years. Despite the craziness in the wake of the election, I was still able to find parts.
It takes patience, but it can be done.
+1 on the upper receivers being hard to find right now. When you find one, jump on it.