You asked for Metro Portland Freqs. Link is for the Metro Portland known Freqs
Ahh.. your looking for the "Metro" being a Bus Svc.....
As for Bus Co's - they are likely
a. Generally on their own Radio System (best to search the FCC DB)
b. On one of the Local DMR TRS Systems (NXDN, IDAS, TRBO) rented from the Local/Regional Radio Shops (They range from VHF, to UHF and 900 Mhz)
FCC Search shows
ULS License - Industrial/Business Pool, Conventional License - WQOQ319 - SOUTH PORTLAND BUS SERVICE
Cumberland County Municipal & Utilities - Scan New England Wiki