Any advice on how to contract out plumbing, electrical and general maintenance repairs for apartments?

I manage 67 apartment units and have all janitorial/housekeeping issues covered, but I'd like to contract out for the mechanical repairs. Any suggestions on how to do that so there won't be surprises when the bills come? I'm thinking there would be a need for multiple contractors to handle specialized repairs (i.e. plumbing, electrical, etc.)