I've ordered from this place before:
http://custom.quality-time.com/home.php

It has been several years and can't remember the prices but they seem steep. As the sheets wear in the torso area, I stitch on a piece of muslin to get more mileage.

As far as your sheets coming off the corners, are they the deep sheets for thick mattresses? If not, you might try some as there would be more material under the mattress to compensate for the change in the mattress when you "adjust".

Another thought would be to get sets of alligator clips with elastic between them that you fit across the corners. This is the idea - not sure if these are for the corners but you'll understand what I mean.

http://www.bedbathandbeyond.com/product.asp?SKU=14808086

Good luck.

I am using the Wamsutta Pinpoint 360 from BedBath&beyond. I was able to buy the sheets individually, so I was able to buy the 2 - twin extra long fitted sheets, and then the regular king top sheet and king pillowcases. BB&B was the ONLY store in Tulsa, OK that carried the twin x-long in individual sheets. In stock. I have never had the corners slip of the mattresses. Ever. The elastic around them is cheap, and not snug, but like I said, they have never slipped off the beds.
They are percale sheets though. I had always slept on sateen sheets so it was a HUGE change for me. For about a month and a half, I was miserable every night. I couldn't sleep on those scratchy, noisy, sandpaper sheets! I was near tears several times. They were the only sheets I had so I HAD to use them. But then one day, they were great. I don't know if it was me or the sheets, but one of us changed and now I love them and will never go back to sateen sheets again. By the way, I wash my sheets at least weekly and NEVER use fabric softener, yuk! They are also the only set of sheets we have for our bed. I hate folding sheets, so I like to just wash and them put them back on the bed. One day I will get a new set (hopefully get to try the sferra brand) and these will become my "emergency" sheets.
On a side note, I bought the mattress protectors at Dillards. They fit the mattresses perfect. The elastic around them is much better than the sheets by Wamsutta. And the best thing about the mattress protectors, they are MADE IN THE USA! The dillards website said they were imported, but the tags on both mattress pads stated they were made in the USA. I did buy them at the store since I was lucky they were in stock!

I have a twin XL adjustable bed. Two together to make a CalKing. I buy whatever sheets I like. I use child sized suspenders to keep them nice and tight.

I've had good luck at Target especially with a sale:

http://www.target.com/p/threshold-performance-sheet-set-solid/-/A-16947681#prodSlot=medium_1_1&term=twin+extra+long+sheets

I've seen the split king adjustable bed done two ways: 2 twin extra long fitted bottom sheets which I've seen at Target and Bed, Bath & Beyond and either the twin extra long top sheet or a king sized single top sheet. Instead of the top sheet you can also use the twin extra long duvet or the king duvet. The twin extra long sheets may not be available in a Target store unless you are near a college dorm, but they are readily available on line and can be delivered to a local store if you need to save on postage.

I don't think a conventional bed skirt or dust ruffle works very well on the split king. The mattress sits right on top of the platform that adjusts. If you use a king bed frame without the slats, the two pieces of the bed just fit inside the bed frame. If you don't want to use a bed frame, you can buy/or build an upholstered or padded headboard, either twin together or a king, that fastens to the wall at the head of the bed. Instead of a dust ruffle, think about a conventional bedspread or comforter with a drop (the distance from the edge of the bed to floor) that covers the whole bed. I have a matelasse bedspread for winter and made a quilt for the summer that makes the two piece bed look like one big bed. HTH

It has been awhile but I believe I got my twin XL sheets at Kohls. If it wasn't Kohls, then it was Bealls. I ended up buying several different sets so I could get the twin xl fitted sheets and also the king size top sheet. They don't come in the same package. The same package contained a twin xl with a twin xl top sheet and pillow cases. So you have to buy a separate king size set to get the king top sheet and king pillow cases.