I mostly collect CD’s and file downloads but also mono vinyl. Roughly 97% of my collection is classical/art music. Arkiv Produktion is a favorite of mine. They are a historical performance practice focused label and division of Deutsche Gramophone. They started with quality mono pressings in the 1950’s and then began pressing stereo recordings up into the 2010’s. Also, Analogue Productions remastered much of the RCA Living Stereo recordings which I believe are a vast improvement over the originals. Other remaster and reissue labels I like are Analogphonic and Speakers Corner. If you want to spend insane money on reissued limited edition classical pressings then take a look at The Electric Recording Company but their runs are extremely short and they sell out fast. It’s best to get onto their email mailing list. Otherwise, I would just visit your local brick and mortar record stores but in my experience, that is very hit and miss. Another recommendation; it sometimes helps to look for composers based on the time periods from which they lived. In other words, Gregorian Chant, Medieval, Renaissance, Baroque, Classical, Romantic, etc… This way, you’ll know the period or style of music your interested in and can discover works by composers that you’ve never heard of. This is how I found works by Zelenka. And lastly, you may benefit by getting a periodical subscription like ‘Gramophone Magazine’, or if you read French, ‘Diapson’. And, they of course they have websites.