I have seen mention of Devcon 5 minute epoxy on this forum, I recently used Gorilla Epoxy 5 minute with success and there is one other that has the 2 separate bottles called Stick Fast. Walmart most likely will have Gorilla products.
If it's your first time glueing a knot in it is easy to use too much epoxy (or too little now that I've scared you about using too much). Have a plan in place to clean up the stuff that squishes out. Do a dry fit before you attempt to glue so you know how far down is "all the way". It may be totally unnecessary but I will often cut the corner off a plastic baggies and push the base of the knot through leaving the hairs covered in plastic. This just keeps me from getting epoxy in places I don't want it.
I think any two part epoxy will do but I like the 5 minute stuff (loctite has some that works well). Make sure you mix it well and don't overfill the hole. I try to coat the sides top to bottom and dribble a little at the bottom so when I push the knot in it slides easy and I am pushing the epoxy down instead of waiting for it to squish up.
Any water proof epoxy will be fine.
The 24 hour epoxy is about 2x stronger than the 5 minute kind. Not sure it makes any difference for a brush though
You can even use silicon caulking for a week or two.
Ps: just use a little bit. It squishes out onto the lower part of the knot easily. The knot still works fine. It just looks ugly...