For the shows you've previously announced, make sure the show managers know that you're interested and available for their future dates.
If your local circuit is anything like mine, most of the shows are managed by the same company who have their usual go-to people to run the gates and office. But if someone gets sick or has a conflict with another show, there may be a last minute opening. Again, talk to those companies/show managers and let them know your interest and availablity.
I've filled in several times for last minute illnesses, but only because I've built relationships with those managers and encouraged them to call if they need help. Good luck!
I manage shows - just come up to me and you've got a job!
Hi Hunter's Mom...
Where do you live? As a show manager of a local series (non-rated) I am totally up for folks approaching me and saying "Hi, do you need a helper today/tomorrow/next week/next show?". I either tip or swap out their kids entries but some volunteer for the free cookies. Don't ever be shy about offering your services - just be upfront in what you want out of the deal. Announcing is super fun and makes the day go by fast. Good for you!
I would check out any riding clubs, associations, barns etc that hold shows in your area find out the contact information from website or email address - email them - good announcers are hard to come by so it's always a plus to have more names.
My uncle announces all over New Jersey and just introduces himself to people when he is at the show if he knows that they themselves run a show. Also, he likes to come to my horse shows and when there always introduces himself to show management. He has picked up a lot of work that way.