I have been buying obscure (and sometimes very desirable) records for years. Short of the FMU show in NY and one year at the Austin record show (still have not made it to the famous show in the Netherlands), almost never find this stuff in record bins. Discogs usually, sometimes, private sites of dealers. Condition is key, and grading these days is loose. I find certain vendors who I will tap into til the vein runs dry or their prices get nuts. There are different vendors who specialize in different genres- for some years, I was buying up OGs of spiritual jazz until that got to be crazy money, especially for the private label stuff. Ditto on the OG Vertigo Swirls, particularly from the UK. The market is still high despite general economy depending on what you are looking for. I've slowed way down in buying b/c of this- and some of the records were simply hard to find- e.g. the OG of Alice Coltrane's Ptah had not been reissued after 1974 until a recent reissue pulled from a digital master. It took me a few years back when to find a clean original though it wasn't "rare" as such- an Impulse release.
Best approach is dialog with seller unless you know them. Get a sense of what they know, what their standards are, right to return (I hate returning records and explain that). We are talking about some records that now sell in the 4 figures, but I bought them before they hit those prices. Good luck, the hunt is part of the fun.
