I think Patrick is in the Philippines until April or May.
Almost any truck wrecking yard will have the transmission that would work well for you. The nice thing about all of the Eaton Fuller transmissions that were used by Crown and Gillig is they all used the same universal shift tower mechanism used in Class 7/8 trucks. In other words, outside of changing the length of the driveshaft or changing the universal joint it shouldn't take much work to swap a transmission with OD into your bus.
If you are not going for an OD transmission the question has to be asked why would you bother making any changes?