After searching and searching some more, I found a couple cases that work very nicely for the SL5500. I'm a newbie here, and saw one of these listed, but not the other, in the forums.
Anyhoo- both of the cases are Case Logic brand, and both are black pseudo-leather.
One is a slip case with belt loop that I saw mentioned here- it's model PLTK10, and is CompUSA SKU 304230. Price at my nearest (150 miles away!) CompUSA was $14.99. This one is nice and snug, but not too tight, and has an elastic catch strap on the top. There's also a sporty version of this one made of heavy cordura nylon, but I didn't save the model number or SKU.
The other is a zip-up case, model PLT2, with room for some credit/business cards and the like. It uses the "Strongman" plastic clip-arm mounting instead of the stickyback velcro, keeping you from having to put glue on your Z. It holds nice and snug, but you have to remove the standard Sharp flip-cover from the Z. Added bonus is that a very quick mod to the bottom fingers of the mounting (trimming 1/8 to 3/16 of an inch from the clips) allows you to slide out the keyboard for use. Cool! Price for this one at CompUSA was 19.99, and the SKU is 290411. This was the last one at my store.
(disclaimer- I have no affiliation with CompUSA or Case Logic)