Peter won't send badger knots out of the country as there are restrictions put on commercial companies regarding the export of animal hair. He's stated several times that he won't do it and to not even ask.
You could always order just the handle without a knot, have a knot delivered by someone else and install it yourself. I'm sure you could get a micrometer and give Peter the knot dimensions including the exact size of the plug and what you want it set at. Either install if with some silicone if you want to remove it again later or some 5 minute epoxy for a permanent solution. It's pretty easy to do.
For Peter's brushes I like to use a Shave Envy Luxury knot (around $44) or a Shavemac 2 band Silvertip (not the d01 version) at about $120. These are both Silvertip badger hair.
As far as synthetic knots go I have no experience with them. I'd share with Peter the attributes that you find desirable and ask him for a recommendation since he's likely familiar with all of them.
+1