For those in the Bay Area, the SFMOMA has a good number of Thiebaud paintings and sketches on permanent (I think?) display including a number of his figures and landscapes (not just his more well known “pop” subjects and still lifes). And, unless it has changed during Covid, you can visit Crown Point Press in SF where Thiebaud worked a number of times over the years (as recently as 2019!). They have a lovely gallery and shop, and may even let you wander through the working areas.

Edit: And I’m reasonably sure his gumball machines painting is in the lobby of the de Young museum, where you can see it without purchasing a ticket (though you should as the de Young is awesome).