"sport briefs are my favorite style underwear and I have the same problem. I have noticed they are very regional and sporatic where they are found. Here in my hometown, I can't find them anywhere, at either high end stores, nor at discount stores like Wal-Mart or Target. (then again, it took this area a long time to get skinny jeans in stores too) I frequently do work in Washington DC, and I noticed the Macy's up there have them frequently. I found the 2xist Sliq sport briefs one of my favorites, and have bought several pair from the Macy's at Pentagon shopping mall. I was in Charlottesville VA a few days ago and stopped at a K-mart, so I decided to check out the underwear there. Surprisingly, they had a nice variety of "Joe Boxer" bikinis and sport briefs. The K-mart here has a very limited selection of underwear and the briefs are limited to the "tighty whitie" styles in various colors.
One thing that is becoming hard to find is briefs without the large, wide waistbands too. Being slightly autistic, clothing tends to irritate my skin. As a kid, I wore Jockey Elance briefs all the time because they have a thin waistband the wouldn't irritate my skin (The wasitband on Elance briefs is similar in size to the leg waistbands).
I am not sure why this is. I guess there are regional preferences, and the stores simply stock what sells in one place and not in others. It's kind of annoying that my hometown doesn't know what good underwear is.