I'm not in your area but there's a lady comes out to our yard with one. Costs £20 for a half hour session. I think her basic course is once a week for six weeks. Don't know if that would do if there's someone in your area.
Try Ebay. Put in search of Niagara massager and there's one with 2 days to go at £26 at the moment. I have one and it is exactly the same as the "horse version" I think I paid £50 for mine and it has been brilliant and all the horses love it.
I know you were looking to hire one but it mat be cheaper to buy