I think #3 would be the easiest. I used to make fitted flannel sheets for a Chiropractor's massage tables(the ones with the nose holes). I measured the length and width, added the depth/height, and added a few inches for the underside. The mitered corner is just a cut-out of the last two measurements(with a 1/2" seam allowance), so the sheet is a rectangle with squares cut out of the corners. You simply line up the inside edges of the cut-out corners and stitch them, then attach however much elastic you want to the edge of the sheet. For the Dr's sheets, I just pressed under 1/4" and attached 1/4" elastic to the entire edge using a zig-zag stitch.
You could always use scrap fabric and make small sample 'sheets' to get the hang of it, maybe using an end table or ottoman for measurements. HTH

I agree with noinwi, the third solution making it yourself would be the easiest way to go. Do a google search for how to make crib sheets and there are lots of directions and pictures on how to do what noinwi says about the corners. I made crib sheets and it is a snap to add the elastic after your corners are sewn. If you have a serger, you won't even have to hem the sides. Just serge and then attach your elastic. Your measurements would be different of course but the method would be the same and it is an easy project.
Good luck!

Another possible solution....there are "garters" that can be used on sheets that are too large, to gather the sheet together at the corner. You can probably get the garters at a place like Bed, Bath and Beyond, or perhaps Sears.

Donna

gosh..I wish you lived in VA..I could do this for you. It's easy and I've done this many times. IF the corners are simply boxed.. (sq corners) You can cut off several inches all the way around and then rehem and run new elastic. I also work in a shop doing clothing alteratios, worked there for yrs sub contracting work.... but any decor type work is simply offered to me...so I do it on the side.
You can also use a flat sheet and make fitted ones. That is easy too. Simply lay it on the bed. EVENLY. Allow extra for hems, and to "gather under a bit.... Cut pin corners in Cut off excess, leaving some to sew. I sew mine twice...once on the Right side, then trim, flip and sew from back making a nice finished seam,,,,then hem edges all the way around and run elastic.
OFten I just run elastic across the bottom, top...and then only about 12 inches down the side.... no need to go all the way down the sides