Any flying school that is authorised to do both Instructor Ratings and multi-engine training can do a Gr.1 & multi-training approval.
Some will have an in-house testing officer, others will rely on external ones - ATO or CASA but usually ATO.
Cost is somewhat variable. My Gr.1 cost about A$300 - A$500 in 1991. My M/E trg. approval cost about A$1000 in 1992.