I just bought a pair of photographs from eye buy art and custom frames from American frame. Lots of sites are around like that. Photography may be easier than paintings depending on how picky you are. Etsy is another option. Auctions, estate sales etc.

But honesty IMO you need to expect to spend something over $1,500-$2,000 framed at least to get something decent and the sky is the limit.

Commission a student artist at your local art college?

I rarely buy framed art I like just the canvas stretched or something very simple in a frame. I agree with grandebouche , check out art colleges in your area , find an artist's work you like and see about getting some custom work done.

Perhaps think outside the box. Find something or lots of somethings that could be framed, for instance and just off the hip; colorful wrappers, or faux butterflies or magazines. . Good Luck!

Go to local art fairs and get the names of artists in your area. There are often artists who are not yet well known or are just starting out and looking for new clients for their portfolios. You may just find someone whose style really resonates with you.

Since you live in San Francisco, you may want to consider an authentic street banner from one of our local museums, SFMOMA, de Young, Asian Art Museum, Contemporary Jewish Museum, or Legion of Honor.

We at BetterWall represent these plus another 30 art museums selling their authentic street banners to people to put in their homes and offices as spectacular wall art. As unique objects produced in limited editions, the banners embody great art, great museums, and contemporary advertising trends.

Priced between $300 - $800.
We have over 120 from which to choose from at www.betterwall.com

Here are a few examples from SF museums. The recent SFMOMA Diebenkorn exhibition, plus Robert Bechtle, and then Degas and Yves Saint Laurent from the de Young Museum.

Since you live in a very wealthy area, keep your eyes out for garage sales. My son and DIL have a couple of HUGE pictures in their home that I assume they got at a garage sale as they also live in a very wealthy area.

Check out duke.paints here on Houzz. Very popular.

yes, i have had a few from garage sales, better is estate sales. i also do charity thrift and auctions and antique stores. remember these not only for your artwork but your frames. yes, if it is oil or the all purpose, i do not frame. only water colors etc. other places i do well are etsy and saatchiart.

Etsy for sure

Etsy is where I found my large canvas.

zgallarie has some very large pieces as well as High Fashion home.