Hey Rall - first of all - welcome to the forum! Glad you found us. If you get a chance, come on over to the new member forum and introduce yourself.
On the sheets - here is what I do -- on the bunks - I just use the closest size fitted sheet and then tuck the excess under. The weight of the bodies seem to keep it in place and I have never had a problem. My bed is a short queen. I use regular queen and then use those elastic straps (or sheet suspenders) made for sheets in all four corners. It gives me a nice tight fit and I dont even notice they are too large. They would work just fine for a bunk bed as well. I think I got them at Target for around $5.