CareMedical lists walkers and rollators on their list of rental equipment available. The contact info is in the disABILITIES FAQ sticky near the top of this forum. You may find other vendors with an Internet search.
Rollators can be bought on Amazon for $100. I know because that is where I got mine. They are easy to put together. Mine has a seat, under seat storage and a seat back with more pockets/storage. You can rent if you want but for probably not much more you can keep it and have it when you need it.