In some ways the problem with Grassington (apart from it's small size, so just not a large enough base for tours) is also that any tours from Grassington would only be of filming locations, as Grassington and area is "only" the filming location for the most recent TV series.
That isn't to say that Grassington is not a great introduction to the scenic beauty of the Yorkshire Dales, because it is. It is a splendid area to visit.
"Helen's Farm" is one example of that- it is just make believe as Helen was a Solicitor's daughter from the Cotswolds- then an office worker in a mill in Thirsk when they met, courted and married. She had no connection to any farm. That was a deliberate work of fiction for the original books because there were strict rules "James" had to work around regarding a working vet writing about his life.
Yet the TV farm is as good an introduction as any to what a Dales farm is like. The location scouts did their job well in that regard.
I hope you have got The World of James Herriot at Thirsk on your itinerary (which was his authentic surgery)- train or bus from York.
But the "authentic" area "James" worked in was really Wensleydale and area- the original TV series and movie were much closer in their settings to his "real life". In fact like the modern surgery the area also included the Hambleton Hills to the east of Thirsk. That is why one of "James" favourite views was Sutton Bank (east of Thirsk towards Helmsley).
Even Skipton (the much larger market town for Grassington) doesn't have any tours of the area I am aware of.
On a Summer Saturday and Sunday there is a direct service bus from York to Grassington- the #74 on Saturday at 0823 from York Station, and on a Sunday the #822 at 0930, and the #875 at 0820.
