Hello Bob -- Welcome to the Forums!
Don't know about your exact floorplan. If the beds are in the back corners maybe just one corner of the mattress is rounded? If so you'll need a custom mattress. There is no common national source for these that I have heard of. Everyone's hometown has several custom mattress manufacturers -- a number of members have spoken about finding good product & service at these. An inflatable (but pricy!) alternative is http://www.bearbeds.com/airstream.html
-- search on 'bearbed' to see some member reviews.
This length twin is similar to long twins in most dorm rooms. Check out J.C. Penney's for XL twin sheets. We use them unmodified.