Where do you look for rare records and pressings ?


Reliable sources, people that got or can get real things. Not ebay, discog, craig moerer, garbage bins, goodwill, yard sales,thrift shops etc.
Miles Davis, Mahavishnu Orchestra, Dead Can Dance, El Camaron/Paco de Lucia etc -  pro and test pressings, including first release Japanese. Couple of Pink Floyd and Deep Purple maybe.
I have a few, they sound vastly superior. Best way to improve the sound.
inna
I’ve had consistent good luck at antique malls locally. There’s always a good-sized selection from a number of dealers, who must have no other place to sell. I came close to purchasing someone’s well cared for lifetime collection of classical albums at a dollar a record there once. I found them in the back corner of the antique mall. After discussing the purchase with my wife, I went back the following day and all several hundred were gone. Maybe a dealer got them. That’ll never happen again.

Mike
Aside from select Estate Sales , we have Vintage Vinyl and recently, CD Warehouse here in St Lou that have hidden gems if you want to put in the time or days to browse.  I have been lucky on finding some Japanese JVC Direct to Discs, EMI Toshiba Pro-Use DD, and some extremely well made GRP Digital Masters.  Stumbled upon a very rare Ray Manzarek sealed copy of the "The Golden Scarab"  last. It is his first solo album and I have my open copy to listen to.   
If you can locate German audiophile universe, you will find a lots of esoteric/rare stuff. They are serious about it. There used to be a few small stores in Amsterdam that were well-stocked, but they have watered down over the years.
I heard that German audiophile society can be a source for master tape dubs, not sure about rare records. Besides, I don't speak German, though a number of Germans speak English.
Discogs, WFMU's annual record sale (though I've only been once since I live on the west coast). I'm also fortunate enough to live near several excellent record stores.
I did get a few from Japan on ebay but that's it and I can't check ebay every hour in an effort not to miss anything. Same with discogs. We just need more upscale vintage boutique stores, not only for records. I liked ebay much more in the beginning when there was no buy it now, only auctions. It was a little wild and sometimes you had to send cash. No problem for me, I lost some but gained more. And there was virtually no overgrading, in my experience, and Japanese usually downgraded.
Yard sales pretty much dried up around here in N. Va twenty years ago, after a rapid decent. And it used to be so great for a long time, maybe the best in the country. I could hit fifty yard sales a day on a good day. Maybe it’s all the recessions, but now it’s only dirty baby diapers. eBay is the final frontier.
Yeah, chase never ends. Since I don't need many records I have an advantage when it comes to the price, within reason. Couple of records a year is fine. Trusted dealers is a good idea, those that kind of accept special orders.
By the way, I ignore amazon as much as I can, it is a monster company closely aligned with the government, not the best part of the government. I don't care what they got.
Most of us that have collections built over decades have some simple rules, like buy it where you can... and never pass up the one you want, as you may never see it again.
There is no easy to flip switch that makes for ease in getting to a bigger collection. If there was, we'd have all marched through that door ourselves.

You gotta work at it. For a long time. No shortcuts except that of money.
Discogs, unless (once in a while) my local record shops have what I'm looking for, and I try to support the local guys whenever possible.
There are some trusted dealers out there, but no assurance that they have what I am chasing at the moment and prices tend to be higher than what I can find on the usual sources like E-Bay (country of origin in some cases) or Discogs. Every once in a while a general web search will yield gold. I found a dealer that had bought out Lloyd McNeill's old stock of records- the early ones were on a private label and the masters don't exist any more. Finding those in the wild was close to impossible. Sometimes you just get lucky. 
That WFMU show in NYC is about as good as it gets in the US as far as I'm concerned. Haven't been to Utrecht though. 
I also have a friend who is a serious buyer of records- he has his own labels and is a distributor, but spends his free time chasing records. He finds all kinds of stuff and will include me in his buys-- winds up turning me on to a lot of stuff I never heard before. One of his favorite sellers had a table at the FMU show that I culled through a couple years ago. I did not recognize ONE record! Obscure japanese fusion/jazz. Some of it isn't necessarily my cuppa, but it is always interesting to push the boundaries. 
Believe it or not, I've gotten rare records from Amazon sellers! :)
Personally I find what I need on Discogs. But you may be looking for a trusted dealer with a carefully curated stock, perhaps somewhere like Brooks Berdan which keeps a selection of audiophile approved records - you will pay top $ however