I would completely agree that Dave Weber would be the go to guy for minor league singles. That said, as DaClyde noted, Dave is currently not fulfilling orders…and I’m getting more and more concerned that he may have passed away because that message regarding his health has probably been up for close to a year. I would order from him several times each year and the last order he fulfilled for me was around July/August of 2019 - and he typically processes orders within days of receiving an email. I sent him emails for potential orders in February and April of 2020 and never heard back. In April 2020 that message about his health was not posted to his site, but I’m guessing it must have gone up at some point not too long after. I have sent him two brief “I hope you’re doing well” messages in the last 6-8 months and no response to those either.
I also have been a long-time customer of STB and would highly recommend them. Over the past few years, Steve (the owner) has been well aware that team sets don't seem to be commanding the $$$ that they used to and has reduced prices on a good amount of his inventory. A lot of sets are still regular price (which are still very fair), but there are some great deals to be found.
One other long-time minor league dealer who you might want to consider is Lake Country Prospects out of Minnesota. They used to have a website as well as a semi-annual catalogue, but the catalogue was discontinued a couple years ago and the website was taken down at some point in 2020. If you want to reach out, you can email minors@tekstar.com. But please note that he only deals in complete sets and you would need to specify the teams/years that you are looking for and he’ll respond back with prices. (And I did check to make sure he was okay with me posting his email on this forum.)
And a final place that might be worth checking is Beckett Marketplace. I’ve had decent success picking up minor league team set singles from the 80s-2000s from a few different dealers on that site – the best experience by far has been with 7th Inning Stretch.
Hope that helps!