先 here is short for 先至
先至 (first then)= till... then
I think if we wait (till) tomorrow (then) to buy ticket we will certainly fail to buy any
The word "等到" (wait) is implied
Example of difference between "先至/ 先" (till...then) and "先" (first/ for now)
做好功課(先至/先)准睇電視 - (Till) finish all the homework (...