Wittering Grange have a notice board, and lots of knowledgeable locals who may be able to help. They do livery, but while I was on base there, I kept mine in Colleyweston. There is a saddler in Stamford that has a notice board as well.
There is a nice yard called Grange Farm it has small indoor school, outdoor school, walker and is the national headquarters for jump cross. I've no idea on pricing etc but it would be worth giving them a ring their website is Grange farm leisure.