You have a product aimed at physiotherapists. Find out where they spend their time (forums, email lists, etc). Most niche forums are operated on a tiny budget,

You have a product aimed at physiotherapists. Find out where they spend their time (forums, email lists, etc). Most niche forums are operated on a tiny budget, so you may be able to buy advertising for very cheap (perhaps in the form of a "sticky" topic). Find these places, and get in touch with the admins.