Check who is playing on the pitch prior to and after yourselves, it might be that they have an umpire or a player/umpire who may be willing to help. NB Please do not approach West or County appointed umpires whose match is after yours. Appointed umpires are told that they should not officiate matches before their own appointment.
Build a rapport with another local club who play at a different time to yourselves, it maybe that they have umpires or player/umpires who are able to help you. It works both ways.