If you want a rubber one the 801 tip off a cartridge works. The 801 drip tip works very good. I like the aluminum whistle tips, you can get them at Nhaler or Ikenvape. Nhaler has them in 11 different colors but Ikenvapes are a little cheaper and have them in silver or black.
If you push the regular rubber tip in past the stopping point it will stay on much better. It is a little hard to do the first time and you have to twist and push but after that it is easier.
You can get the 801 carts here for 2.95 for five with free shipping.
Empty Cartridges
You can get 801 Aluminum Whistle Tips here
Whistle Tip Drip Tips
They both have regular delrin too.