Yay! While doing a google search earlier today, we found that DS1's therapist had joined another practice (one that we had been considering when we originally found her).
I couldn't be happier. Not only will he be able to get his therapist back but we were able to keep his appointments on the same schedule (awesome for my routine dependent child), start again immediately, and our commute has been cut from 45+ minutes one way to 11 minutes. (Which means having to pay considerably less each month for babysitters for the younger kids.)
When we told DS1 that we would start seeing her in a different office he said "I should bring my chess board in case she forgot to bring hers when she moved."