Although I have been only playing for about 2 days I know the location of just about every ticket. You will need a lot of hustle if you want to get them as fast as I have.
Custom Ticket A: You should get this ticket when you defeat Blooper at Palms Practice Range. Hits needed: 2
Custom Ticket B: Get this by defeating Pokey at Dunes Practice Range. Hits needed: 3
Custom Tickets C: There are 2 C tickets. To get the first one, defeat Monty Mole at Links Practice Range. 3 hits are needed to take it down. The second one is acheived by completing Coo Coo Course with a par or better.
Custom Ticket D: Get this one by talking to Kid once you defeat the Mushroom Tourney. You will find him at the Links Practice Range in the Driving Range area.