listed on pg. 118 in the book "mathemagics" by arthur benjamin et al. it's based on phonetics: t,d-sounds=1;n-sound=2;m-sound=3;r-sound=4;L-sound=5;J-sound=6;k-sound=7;f,v-sounds=8; p,b-sounds=9;s,z-sound=0.
there's a clever association given each for mnemonic purposes: "t" has a single shaft (as does "d"); "n" has two; "m" has three; "r" is the last letter in "four"; "L" is the shape between thumb and forefinger of the open 5-fingered hand; "j" is shaped somewhat like a reversed six; part of "k" is shaped like a seven; "f" in cursive resembles eight; "P" is a backwards nine ("b" an inverted-reversed one) and zero begins with the "z" or "s" sound. there you have it. i can recommend the book: part of my permanent library.