I have had great luck picking up used cabs from Music Go Round or some other used seller. I gut the cabs, then put my own guitar speakers in.
I got a used Yamaha SV15 PA cab for $99 bucks, gutted it, left the holes as ports, and put a 15 inch Weber alnico Blue Dog in it. It sounds awesome!
I just found a used Peavey T112 column monitor for $40 bucks. Great cab, built like a tank, gutted it, put an Eminence Commonwealth in it, and it sounds awesome.
I have even bought old non-working 1x12 or 2x12 combo amps just to get the box, always under $100 bucks. Remove the guts and junk speakers and put new Eminence or whatever in them, they usually sound great then. No matter what, I get a factory built cab ready made for whatever speakers of the day I am test driving. They are a heckuva lot cheaper than the new cabs will ever be, and a lot of times they sound similar or just as good. Cant build one myself for $100 bucks.....
Sometimes, I even get lucky and find solid wood or birch cabs unfinished for a decent price on ebay.