not totally what you are looking for, but the mini milk bun from salty tart is super rich and delicious. Otherwise, good brioche has eluded me, unless you just learn how to make it yourself. Turtle bread makes a loaf sometimes, just ok, though.
I have made brioche buns a few times of late. The problem with them is that they don't keep for more than a day and they are expensive to make. I haven't seen any out. You might try to commission some from Rustica - ?