I usually run rounds workshops for adults at "Folk Camps". Children are welcome too, but we always sing a huge variety. Once people know you like rounds they are always coming up and saying "Do you know this one?" so it is quite easy to build up a collection!
One that is fiendishly difficult is 'Fie, nay, prithee John' It takes a while to master, but it's worth it! There is also a 6 part "Sanctus" that some friends and I have been trying to get right for about 8 years!
I rescued an old book of rounds from one of the schools I used to work in. The room was being cleared out and I took it before it went into the skip. It has some standards like 'Dona Nobis Pacem', but also lots of other stuff. Although I can't scan into the computer, if you would like a hard copy, I could photocopy it for you - PM me if you want. (I'm sure it's too old to be covered by copyright!)
One of the joys for me is when I learn a new round and only hear the overall effect of the harmonies after I've taught it to other people, it's always a lovely surprise!
Love Sue vG