Combination of my local second-hand bookstore (which has a wildly good selection given that I live in a small country town), my two favourite "regular" bookstores, Libby, and the Kobo ebook store.
If I can't find something particularly niche or out of print, I'll use Abe Books but I try to avoid that since it's owned by Amazon.
Edit: I've started to favour print books most of the time, at least for poetry and non-fiction. I've started to write more again and I find physical books much easier to refer back to.