Do you mean the type that slides to the inside of the garage.
The gear is made by henderson it is called ROUND the CORNER
The model for residential doors is the Tangent 290
Go to www.pchenderson.co.uk then sliding door gear/industrial/round the
As to price will depend on opening size and how many door leafs,but it will
not be cheap,have replace the gear on a few occasions and that was always