Best Duets for All Levels?

Greetings!

I have a horn studio of 30 students here in East Tennessee. The studio consists of students at all levels of study; beginning through advanced. In my lessons I usually save the last 5-7 minutes for sight reading duets. The students really enjoy it and their sight reading ability improves greatly.

I'm starting to run out of duets to use, and because I don't repeat duets it's important for me to purchase some more. As many suggestions as possible would be greatly appreciated! (Please indicate the difficulty with your suggestions).

Thank you very much in advance!

Roxanne Haskill