I mean, I practically live at the library. But I use Thriftbooks on occasion for books I really want to read, but can’t find there (I don’t care that much about the condition or prettiness of the book, so I can find most things for like $5 or under).
I have a few audiobook sources: Libby & Overdrive, of course. Spotify has something like ten hours’ worth of audiobooks a month (the time thing is weird because it sometimes cuts you off in the middle when you’re not expecting it, but it’s fine if you plan ahead) and I already pay for that for music. Audible gives you one credit a month and then has plenty of classics and other random stuff “included in your membership,” so you can get lots of stuff without using that credit.
So right now, I’ve got a copy of Fellowship of the Ring from the library and the audiobook on Audible was included in my membership. That one was easy. It usually takes a little more coordinating to find what’s available from more than one source at the same time. And I don’t always combine them. Sometimes, it’s one or the other and that’s okay.
And as for how I keep my place, I just kind of match up chapters and skip around til I find where I left off. It’s not seamless, but it works for me.