Everyone knows COMC, MySlabs and Beckett marketplace, but there are independent dealers out there that have their own websites.
These guys sell through ebay, but if you buy directly from them you can get better pricing and avoid fees.
Here are a couple that I use frequently:
The Battersbox:
https://www.the-battersbox.net/
Extensive selection and fair prices
Diamond Cardboard:
https://www.diamondcardboard.com/
Steve is a nice guy that will sell at fair prices if you make an offer
Stan's Vintage Sports Cards:
https://stansvintagesportscards.com/
Stan has a great selection, and also goes to a lot of shows.
Another online resource is
https://bbcemporium.com/
BBC Emporium doesn't list the cards they have on ebay on their website, but if you find something of theirs on ebay that you like, you can text Curt at 515-779-3889 to get a 10% discount from ebay and avoid fees.
What other independent online card dealers do people like?