Stirling Farmers' Market is held in the Port Street pedestrian precinct, between Albert Place and Murray Place (just above the marker on the map) and outside M&S and the Thistles Shopping Centre.
It's a short walk from both the bus and railway stations to the market and there are plenty of bike racks in the vicinity of Port Street if you fancy cycling along.
If arriving by car you'll find multi-storey car parks attached to the Thistle centre and at Wellgreen Place, good car-parking along Dumbarton Rd and also in the Kings Park area if you are early enough. At the weekends you can also park for free at the Council offices at Old Viewforth and Stirling has various Park and Ride services too - see link below for details:
http://www.stirling.gov.uk/services/transport-and-streets/environment-friendly-transport/park-and-ride-schemes