Ugh, good question. First of all, here in Europe everyone I know either order blanks from http://www.shop-016.de/shop-audioservice.htm or https://tapeline.info/ or overwrite a bunch of second-hand tapes themselves. It takes dedication to into the tape market but I can point you in three directions.

1. The ambient scene: http://fangsandarrows.com/ was a great community, similar to HN crowd. Serious people. Not sure if it's down for good, here's their bandcamp: https://fangsandarrows.bandcamp.com/

2. Vaporwave: If you don't know it: https://en.wikipedia.org/wiki/Vaporwave here's a label https://dreamcatalogue.bandcamp.com/

3. Post-punk / post rock / synth pop (the dark stuff like John Maus) etc. Concrete basement shows etc.