If you don't have a local gun store that carries lowers (or if they do they are probably all sold out), just do a google search for whatever lower you are looking for. If you search for "AR stripped lower" you will come up with all kinds of sites that have them. They will probably say "not in stock", or "backordered". Or you can search for "stag stripped lower" or "kaiser defence stripped lower", etc. You will need to also do a search for a FFL dealer that will accept the lower for you if you find one to transfer it to you. Will probably be charged in the area of $25 by the dealer to do the transfer. You could also look on Gunbroker. They have a lot of them there. You will have to do a transfer from them also.
Right now, if you find one, and there ARE plenty out there, the prices are getting kind of high. This is due to the demand for anything AR right now.
Just do some searches, you may get lucky and find one at a decent price. Good luck on your search.