Make sure you get a firm commitment a month ahead of time. Plan out hotel and travel arrangments with this person so you are sure he is ready to go. If you get down there without anyone, be prepared to take whatever you can get. Make sure someone in the group has a cell phone with lots of available time because that will be your only way to contact potential subs off the board.
Last year, we lost our best bowler due to mother's illness and eventual death. Obviously he couldn't come. We drove down there not having anyone. Turns out we found a superb sub we knew who was working the tournament. He hadn't bowled in a couple years, but was a good enough bowler that he found his groove easy enough. We were checking the sub boards and couldn't believe how slim they were as we bowled in June. You can get lucky, just don't expect much if you're looking for a last minute sub.