I presume purchasing two 9T cats isn't acceptable.
Short, simple method: purchase a 9T which matches or is recessive on all 8T you presently have. Breed with the 8T until you get an 8T which can mate to the 9T (on average: 1.5 boxes required). Switch the new 8T offspring for its 8T parent and breed until you get a 9T (on average: 3.5 boxes). If can mate with 9T parent, you're done. Else if can mate with 8T parent, idled 8T from OP, or any box from this post, start another line. This can give you a large number of 9T lines (in theory, one line for each possible combination of traits present).
Longer, harder method, purchase an 8T which shows the missing trait. You only want to fall back to 7T, so set aside any 6T boxes. This increases the process because you'll have to cross 7T hiding 9T lines, to make 8T hiding 9T, and cross those to make 9T. On average, this should take 9 times as long as above.
Even longer, even harder method: same as above but purchase a 7T, work up from 6T, setting aside 4T and 5T boxes. This should on average take 9 times as long as the method just above, and 81 times as long as purchasing a 9T.
Even longer, even harder: get a 6T , work up from 5T, discarding 4 3 2 & 1 T boxes. 729 times as long as purchasing a 9T.
Even longer, get a starter (0T), 1, 2, 3, 4, or 5T and, basically, start over. You won't really be, but you'll be making a chamber-pot-full of 0T boxes so it's gonna feel like it.
