Still get some sort of screen protector, even if it's just the plastic kind- you still run the risk of scratches from bits of dust n' such that get on the system before you close it, not to mention using the stylus itself.
As for a case- mine's in a old DSi XL case. They're the same size, so anything designed for that should work on your 3DS. (If you're feeling cheap and can sew a straight line, crew socks are easily converted into slipcases for tech.)
Edit: Just now caught that it IS a DSi XL you have. You really can't find a case? Are you looking for a specific style? I didn't really have any trouble finding mine- but anyway, yes, a 3DS XL case will work since they're the same size. Make SURE it's for an XL, the older 3DS is notably smaller and the original 2DS is a completely different size.