Marks Firearms is selling some Spikes lowers and he is close to you. https://www.okshooters.com/showthread.php?196283-WTS-Spikes-AR15-Lower-Receivers-and-Complete-Spikes-Upper-receiver-groups
I just finished a non polymer budget build. If you are patient, Palmetto state has stripped blemished lowers for $49.99 and a DPMS parts kit for $59.99, it'll take a few weeks to get here though. You can get a mil spec size buffer assembly on ebay for $20 shipped.
That's the cheapest route to go and still retain some reliability IMO.
Joebob's would be my next recommendation, they have a wide range of prices on their lowers, lpk's and always ship by the end of the next day.
If you want it now, I'd head up to Sooner State Pawn, you can check online to see what they have in stock. They had complete lowers for $199 not that long ago, just switch out the cheap stock and all that and you'd be done, though that's not nearly as fun!
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