Any of the major places on the sponsors list – smokingpipes.com, pipesandcigars.com, Blue Room Briars are all good places to look. Also any of the ones on estatepipescentral.com . Pipestud is another stellar source -- one of the best for quality, although his supply of estates goes up & down a lot. He’s one of the very VERY best is pipestud. His rep is beyond stellar, & justly so; you can buy from him with 0 worries at all. You'll also find him on eBay. Speaking of fleaBay, there are more than a few that really stand out as 115% reliable & trustworthy. I hope I don't neglect anyone here by *not* mentioning them. Good ones I've bought from (in no particular order): colwright; twestpipe & kahuna100 (I've had several good experiences with all 3); fory63 (in Germany; I've bought 4-5 from him); & bima1960 & pipesonrattan (both in Italy; also multiple happy buys from both).
As far as good brand choices, Savinellis of course are always good as estate pipes, as are older GBD,s & Comoys. Their current stuff may not be *quite* as high quality as before their acquisition by megacorps like Cadogan, et al., but for everyday smokes they're still good. The real bennie is that good relatively low-priced choices from "back when they were REALLY good" aren't that hard to find, GBD in particular.
[[If you see a GDB that has a little oval brass logo in the stem, it means it was pre-Cadogan & very good quality. A similar key for Comoy's is if the seller mentions something like "3-piece [or "3=piece"] C." It was a complex inlay process that used 3 drillings, a circle for the white C, a second smaller one for a black inlay inside the letter, & the last for another black inlay to open the letter up. Yeah, serious, they were that thorough; shows you the care they put into their pipes back then. When Cadogan bought the company they went to a single inlay, & later to a laser imprint. Doesn't mean they're not good enough, just that they're not quite AS good as back in the day. You can find some good info on it at pipedia's Comoy's entry. https://pipedia.org/wiki/Comoy%27s_Dating_Guide. One last word on Comoy: if you can find one of their "Golden Grain," "Pebble Grain," or "Blue Riband" models for a decent price & in decent shape, BUY IT!!!!!! TRUST me, you will NOT be sorry!! ]]