Sun Belt Rentals rents engine hoists (cherry pickers) and transmission jacks and some other specialized repair equipment. In most major metropolitan areas, there's also local competitors in that market.
I've used Sun Belt, and a local rental place, to rent a higher quality transmission jack. I used to own one of the inexpensive ones (got mine from Northern Tool, IIRC). I had a friend borrow it, and he wanted to keep it and gave me cash for the price of one at Harbor Freight. I do so few transmission jobs now (I'm semi-retired), and the quality of the offerings at Harbor Freight just wasn't impressive. The equipment rental places have "commercial grade" (big, strong and heavy) jacks for rent. The last time I used one, it was for an older RV with a big Alison automatic transmission, and I think I paid $35 to rent it for a "day" (over the weekend, from end of day Friday to early Monday morning usually counts as one day at these places). That was about 5 years ago, and I'm sure prices are higher now, probably for the exact same jack I used.
