There's no point in the ball having such a small chance of turning a monster shiny, let alone making it hard to find (so also unbuyable, I presume). At that rate it's about as rare as getting an already shiny Pokemon. Might I suggest instead a 1 in 25 chance and that it be attainable in the same manner as the Premier Ball? That would be to say, per ten ultra balls (which are relatively expensive) bought at a certain ship gets the player one shiny ball.
This is untrue, it's probably as rare as the master ball, and shinies are EXTREMELY rare, so even then, this would probably be only a 1 in about 1500 chance. Shiny chance: 1 in 8500. That's five times the chance. So, this ball is actually very very helpful.
And if they could be otbained as easily as you said it was, the chance of getting a shiny would be around 1 in 300. Too many shinies.
My suggestion is that there's only one, and you get it like you normally get a Master Ball-just a pack with a Master Ball and a Shiny Ball. Here are my Poke-Balls:
Egg-ball: If the Wild Pokemon is in the same egg-group as the Pokemon you sent out, catch rate is enhanced by 2.5 (0.5 better than Ultra Ball, like the difference between Great ball and Poke Ball.)
Partner-Ball: If you're battling two wild pokemon at once (e.g. You and Cheryl in Eterna Forest) then this ball makes a copy of itself and catches both Pokemon at once-that is, if they don't escape. These only are sold at the city with the 2nd or 3rd gym, or right before the first partner-thingy. It costs 3/4 the price of a Great Ball.
Trusty-Ball-Like Partner Ball, it catches two Pokemon at once, but also multiplies the catch rate by two. You can buy these at the Pokemon League and Battle Frontier places only. This costs 4x the price of a Partner-Ball.
Crawl-Ball- 3x boost on four-legged Pokemon.