The only way you’re going to get a lower without a FFL requirement is a private party sale of an existing rifle.
Because the AR-15 lower can be built into a pistol or rifle, the BATF requires new, finished lowers to go through an FFL. The only way around this is to obtain a lower receiver forging that requires machining (ie, it requires more than mere installation of pins, springs, screws, etc - it would need to be finished on a mill, the pin holes drilled out and bushed, etc.)
Lowers that are finished (ie, all the machining has been done) but are lacking the lower parts kits can be had for anywhere from about $80 up to about $130, depending on the quality and various features.
After that, you need to be a tad more specific as to your application and budget for me to give you any more pointers. Completing a lower is a pretty well price-constrained thing - if you get a fully machined lower and put in your own parts kit, you’re probably into it for $150 or thereabouts. Once we start talking of the upper, the barrel, furniture, etc - the options explode and prices start varying widely.
I believe finishing an unfinished lower is a federal crime. You must apply for a serial number and wait for permission to proceed...iirc.