In the UK, this is a rarely done thing (except on turboprops) so very few firms offer it at all.
Arrow Aviation in Exeter do it but you have to fly there. It takes most of a day, about 200 quid.
Well worth doing, especially if you have a 3-blade prop. The reduction in aircraft vibration (with probable improvements in avionics life) can be very significant.
However a small % of engines (esp Lycoming) are so badly balanced that one cannot do a great deal with prop balancing; I've seen this happen.
There is some debate about whether it should be done in flight. Propeller-related vibration is related to prop pitch, and you cannot reach cruise prop settings on the ground, at the cruise power settings.
A lot of people in the USA go for this; it is almost always needed on the 3-blade props which are very popular over there.