Baseball has to have more niche brands than any sport. A perfect example is Benik, a custom neoprene orthotics company that has become the MLB standard SLIDING guard. The crazy thing is that they don't offer just one sliding guard, there's actually at least five different types depending on the player's needs.
Andrew Benintendi wears the Benik W-117B, which according to Benik, has “a pocket with protective plastic shell” which “helps prevent injury to the thumb. A split opening at the forearm and quick-attach Velcro® tab allow for fast and easy on-base application.”
Benintendi's Benik guard can be had at the link via BetterBaseball.com, although at $90 bucks I'd give Benik a call to see if they'll give you a better price direct.