Re: Where to buy badger hair or china bristle.
You want whatever tipping brush you choose to have fine random bristles. You want it to hold up to strong paint and cleaning solvents. You want it to have a relatively soft feel. The stiffer the bristle the deeper the impressions it will make. You want it to be generally well made. If you look closely at a cheap brush versus a more expensive one you will notice the differences. One of the most important is the better brush will not lose bristles even after cleaning in strong solvents. Better finishing brushes have round handles so that they can be spun clean. Take a look at the cut of the bristles on the end of the brush. Better tipping brushes are made for that purpose. The bristles will taper to the tip rather then being a straight cut at their tips. China bristle brushes are made of Chinese boar fur. I misconception is that so-called badger brushes are more often china bristles that have been whipped so that the hairs split making the brush more fine. There are tons of options especially if you order online.
The point is, you want the thing to be soft, tapered, have a fine edge and easy to clean. More often than not you are going to pay a higher price for all those options. The most important, is that it does not contaminate the paint. Trust me, its a real pisser to find a stupid paint bristle dried in to the otherwise perfect paint job you did.

A good tipping brush can be found at Home Depot or Lowes. It may not have all the options I described. A good China bristle will do a good job and they run in the $15-$20 range. A better brush with a round handle, tapered natural bristles in the 3 to 6 inch width start at about $25-$50 range. You can get more expensive ones but then your overbuying.
Hope this helps,
drewp
BTW- a tipping brush should be for tipping only. you can buy a cheaper one for applying the paint. the tipping brush should always have a clean supply of solvent near by because you want to keep it clean as you go along.