In addition to GM, I buy a lot of vintage from The Battersbox:
https://www.ebay.com/str/battersbox, and I've also used MantleRulz:
https://www.ebay.com/str/mantlerulzsvintagesportscards.
I have a nice LCS where I am but they're mostly stocked with modern. Great people and great service though. When I do buy from them it's usually from their Beckett store, and then I drive 1/2 mile to pick it up.
One of the closest other LCSs is the original House of Cards which spawned the Huggins and Scott auction house. THAT was a great store to browse in.
I grew up in Connecticut - anyone here remember Bayshore Enterprises in West Haven?