From recent experience a lot of the horses readily available via the Internet require a financial contribution because the owner is struggling with the financial aspect of owning a horse and seem to see a sharer as the solution to this!
To provide another p.o.v. from a sharer I can honestly say (from experience) if I saw an advert offering free riding I would genuinely steer clear. The rationale behind this is that when I have enquired/been contacted about these sorts of arrangements it has always ended badly - turned up to get...