i got all of mine from sock dreams, high quality, i wear them regularly and they are all in good shape, some i have had for a year

...huh, i don't know why it never occurred to me to ask here, it's literally the best place to find recommendations

Ohmygosh something I can actually help out with!!! I bought several pairs of Kixies brand thighhighs a while back. These have rubber grippers at the top of each thigh-high that help prevent gravity from pulling the socks down. I specifically bought these pairs to stealth girlmode at work and at other places where I'm not quite out yet. I highly recommend!

Pro-tip - if you can, go to a sex shop with a trusted friend/partner/ally and pay in cash.

In case you're interested, here are the socks I picked up:

1 - Standard solid black (NOT super thin)

2 - Argyle-style

Good luck & happy thigh-high hunting :3

Like other commenters suggested, I recommend Sockdreams they have the best quality thigh highs I've gotten from anywhere, and they have great options for taller individuals. If slipping is a concern I would also recommend getting some garters or sock glue (they sell both as well).

For a long time my top recommendation has been to commission some perfect fit socks from a tailor.

After that, I've had decent success with Kayhoma knitted tube thigh-high socks.