Umpire Shift Responsibility
It is each teams' responsibility to fill the umpire assignments with a qualified individual.  If you as a manger cannot perform the duty, you must find someone using the following steps:

1) Ask your team's coaches/parents

2) Ask teams in your division to trade assignments

3) Ask someone from division above or below

4) Ask someone on the Buy-out list (at your own cost)

Emails sent to all managers league wide will almost certainly be ignored
The UIC will not find a replacement for you.