I can't help you with the synthetic knot but I have bought a badger and boar knot from Maggards and was impressed with the price and feel of the knot. P.
If you like boar I can recommend the Jade Omega that has a 27mm knot. It has a lot of backbone breaks in good and the handle is fantastic. I am very happy with mine.
Zenith makes fantastic Manchurian brushes that are two band that are very good value. Might wanna check them out. However, knot diameter is 27mm again..
I don't know if Frank Shaving sells its G4 Knots separately. I just got the 28mm 2 weeks ago and I love that know. Lots of backbone, splay perfect, tips super soft and when it's loaded with soap it doesn't have the synthetic feel but more of a 2band badger.
Alternatively, Apshave co in Canada sells knots (synthetic, badger and mixed boar and badger).