I've knotted new handles and reknotted a few old ones, so these are just some random hints.
Hard to find old brushes in op shops. They say that health laws prevent them from selling them. Actually that's probably not correct but they believe it so no go.
Whipped dog handles are nice but basic plastic resin and the hole sizes can be out by a couple of mm's.
The olive wood handles can be really nicely figured BUT its pot luck. They are not sealed well so they need to be refinished first, I did mine in epoxy resin and they came up really nicely.
Dremel type rotary tool is your friend in removing old knots. Go slow, be patient, wear a mask.
Knots on eBay vary considerably. Virginia Sheng is very good, especially the extra density stuff. She is not user friendly when it comes to questions etc. Ace Shaving is generally a step down but still very good in the better knots. The others should be given a miss.