I started this thread:
https://www.dcurbanmom.com/jforum/posts/list/909255.page
I was looking for soures at lower price point (but still authentic - essentially the resale market place). But there's some good advice in the thread.
I'd start scrolling through EBTH.com - it's an online auction site. You can search by category. There's so much there that you might be able to identify things you like, and you can see how much art goes for. (I see a lot I like there and a lot I hate - it's a giant auction, after all. It's harder to find true bargains there (<$100) but you can definitely get good stuff for <500).
I just recently started looking at liveauctioneers.com
I think I lot fo what they have is mass produced stuff or lithographs/seriographs limited editions. Not as much original art. But also A TON of stuff to look at if you just want to scroll and figure out what kinds of things you like.
I've looked on eBay - given that I've figured out what I like and what I'm drawn to I can use key words to search and there's great stuff there if you can weed through it all.
I'm heavily looking at estate sales. (estatesales.net)
also, art I've collected in the past has mostly come from vacation places - buying a memento. Are there places you like to travel or that have meaning for you? You could look at galleries in those places (online). While they will feature a lot of local touristy type art (like a gallery on Nantucket will have lots of beach scenes) they also carry other artists or other items that might be of interest.
I used ot love to go to Glen Echo and their art shows (the labor day art show was so good, sigh).
Etsy - again once you figure out what you like and what you're looking for, so you can weed through.