Hey,
IMS, Clarke and Acerbis make tanks. You will probably have better luck with the first two. I don't have any personal problems with aftermarket plastic. For the fit, and the stock look, OEM is best most of the time. Price is a factor, but alot of times you have to drill your own holes for aftermarket stuff. So for the extra quality, I'll always go OEM on my plastics.
Obviously, the stock tank for your bike will be metal. So you will have to go aftermarket for the tank. You may be able to fit an OEM XR250R plastic tank, but I have no idea how or if it will fit on your 250L. Check out
www.clarkemfg.com, they have a 250L(all years) tank for $165 (4.2 gallons).
www.imsproducts.com has a '91-'95 250L 3.2 gallon tank for $225. From what I've heard about tanks for XR650R's, most people seem to be happier with the Clarke tanks for the fit, finish, color matching and price. Do some research. Good luck.