In the late Seventies, or very early eighties (I was around ten) We went to Maine where my dad was stationed and stayed in the UOQ there with him (Unaccompanied officer's quarters... used to be BOQ I think, with the B for Bachelors). While we were there, we went to a flea market, and I got a brown paper grocery bag full of comics all in good reading condition. They included Plastic Man volume 1 #11-20 from 1976 or so, and some Omac (1,2 from 1974), and some Omega the unknown and a few other things. It was awesome, and boy did it make my trip. I was right around 10 years old. The other cool things were a bumper pool table (My sister was really good at it), and an unattended health center. 10-year-old me went a little crazy with the apparatus and bike and pulled a groin muscle. That was awful, but a cool bag of comics made it less so.
Surprise comics troves have always been my favorite. there is something great about unexpected free or inexpensive comics, of a type you didn't choose, and might not have ever chosen on your own. I got a pile of Tomb of Dracula the same way, and I have had a soft spot on my neck for that comic ever since. I can't pin down when I got those, but it could have been in Maine also.
My earliest version of this was when we uncovered a box of comics above my grandparent's garage, in a loft, open to the world, on their farm. They had been my uncle's comics as a kid in the early sixties or so, and included things like wagon train, Honey West, Classics Illustrated, and sugar and spike. Those are more examples of things I would not have gotten for myself; from an era and genre's I may have overlooked.
I bought comics at auction back in the late 80's, and I didn't pay a lot. they aren't in great shape, but included an early daredevil, x-men, and the first appearance of rhino in amazing spider-man. That wasn't exactly the same, but it was still a surprise of sorts as I think I only knew it was comics and didn't know exactly what. That one made me feel like I had something, and it was extra special to get to read a variety of comics that I might not have otherwise been able to at the time.
Most recently, my youngest daughter's boyfriend gave me a pile of comics that had been in his garage. His mom is an antiques dealer and sometimes gets miscellaneous things from auctions or estate sales, in lots, etc. It had been in a box in their garage for a while, and he gifted the whole pile to me. It was completely in the category of things I might not have considered, but really loved getting and having. There were 26 issues of Superman Family from 1979 - 1981, and a full run of Time Warp 1-5, and several Adventure Comics from the same era. All of these were oversized "dollar Comics". There is a joy to comics from this era. The stories they are filled with may not be particularly great, but they sure do delight me for the joy and silliness they provide, and the fact that they feel like the era they are from and link me directly to how comics made me feel at that age.