There's a couple of wonderful charities here in the UK for premature babies.
One of them, Ray of Hope, collects all knitted items, little cardigans, mitts, etc., and sends them to hospitals where they are needed.
One hospital might not want little bootees but another hospital might use them, so whatever you've knitted you know will go to the place where it's needed.
I think these days that most bona fide charities would have a website or some kind of booklet/brochure that would give you the info. you need.
All the Best