Shimano still makes 8 speed shifters. Take a look at them. I may be wrong, but I don't think there were any changes to pull ratios and such between then and now for 8 speed stuff. Here are some part numbers you can google to see if any new are in stock. Put Shimano as a search term with each one. Not every one of these will be the type lever you want.
SL-M360-R
SL-M310-8R
SL-M315-8R
SL-RS47-8R
SL-RS45-8R
ST-M310-8R2
ST-M310-8R4
ST-TX800-8R
ST-EF65-8R2
ST-EF65-8R4
ST-EF500-8R2A
ST-EF500-8R4A
ST-EF510-8R2A
ST-EF510-8R4A
ST-EF505-8R
Microshift probably has something too. People seem to like them as replacements for long gone Shimano stuff.