Don’t laugh, as I have had a frustrated visitor to this website looking for long rise underwear as he is a 6’7″ tall man. Here are some options I found in tall underwear:
City Boxers sell long rise boxer shorts. “Our tall rise boxers would be suitable for men at the upper end of 6 feet something.”

King Size Direct also sells tall underwear. 

If anyone else knows of great places to get tall boxers or underwear, send me an email or comment below.