I understand that they are no longer made.
There is a WT member with a wanted advert already, so it is a "keep watching" and check Ebay if you really want the Sato pipes for a used pair.
Yoshis and Mivvs are excellent options and you won't be disappointed with either brand.
Also check Blueflame performance - they have some nice Hornet 900 slip-ons.
Also I did a Ebay UK search last week purely out of interest and there were a number of interesting slip-on mufflers for the 919 there - some nice looking pipes that are different to the normal sets you see on USA bikes. Worth considering if the exchange rate works for you and you want something a bit unique.
Pretty sure you can still get Leo Vince ones too. 2bros also.
IMO don't get hung up on the Sato's for 'performance' as there is a very limited gain fitting slip-ons on the 919, and other than noise levels and tone, you'd have to be a human dyno to notice the difference between brand pipes on a 919.
I'll go as far as sticking my neck on the block and recommend with name brand slip-ons for the 919, buy the ones that physically appeal to your eye and fit your budget.
Heck while on a roll I'll say be budget aware and any significant savings could be put towards improving the suspenders - fork springs specific to your weight and riding etc.
I await a WT verbal beating or group confirmation of my above comments!