Maybe give your local tourist info centre a call? And check the adverts in your local NGS Yellow Book - again the tourist office will have that.

Mr. Google is your friend. Just put in ...plant fairs. Not sure if I'm allowed to put web links on here.

thankyou

Is this any good. You will probably meet other gardeners there who will be able to tell you of other events in the area. if you go to www.wherecanwego.com/Tyne and Wear you will find other events.
Woodland Cottage - Open Garden for NGS
This is a flower arranger’s garden full of vibrant early summer colour. Most plants have been chosen for their garden merit, interesting texture and colour. Small yet packed with perennials, small ornamental trees and evergreens carefully planted along pathways, transforms a small family garden into a layout with stylish design

Admission £3, Children free (share to Multiple Sclerosis) Open for charity.

Woodland Cottage, Whickham, Gateshead Northumberland NE16 4TH
Sun 6 Jun

Yes, Yellow Book gardens do frequently have plant stalls - I know it's not the same as a Fair, but you do get to see the gardens as well.

Hope you find a Plant Fair, though - I really enjoyed the one I went to!