if you are using Apoxie Sculpt, you can use lotion to help smooth with your fingers. Just be sure to wash it really good, and maybe even wipe it down with acetone before washing to help remove the lotion, or the paint wont stick. Also, apoxie sculpt sands really well. Just remember to use super fine sand paper. The higher the grit number the smoother the finish.
Michaels and Hobby Lobby are two chain stores to find paint brushes at. You can generally get 20/0 there easily enough. But you will notice that the 20/0 brushes still vary from brush to brush so you may look through them all for the ones that seem smallest. I get better control out of the 20/0 liner or spotter brushes, but others love the script liner brushes which have long bristles. A cheap brush will leave streaks, and possibly bristles behind. I like to test the softness of the bristles against my (dry) lips to feel how soft it is. These brushes can be expensive, but after you see how well they perform, price stops being a concern when you are after a particular quality of finish. At least, this is how it was for me!
I think I have seen people report finding even smaller brushes online, but I haven't looked myself.