In most of the towns and villages in BL you will find a stall selling pumpkins. In Dornach and allschwil I have seen them.Also perhaps the market at Martkplatz, although I haven't looked for them there.
We went to the Foundation Beyeler, in Riehen (Tram 6) on Sunday, there was a very nice lady with a huge dog selling homegrown fruit and veg including pumpkins by the side of the road. If you get off the tram at the Foundation Beyeler stop and walk about 500m back towards Basel she was on the righthand side.
They should be all around the farm areas. Since it is seasonal, you may have to keep an eye out for make-shift signs. Look for a sign that says Kürbis . I've been on a pumpkin binge lately; pumpkin soup, pumpkin pie, pumpkin curry, grilled pumpkin... etc. Have fun.
There's a great kurbismarkt in bottmingen not far from the hospital in bruderholz...probably had 100+ types of pumpkins, gourds, and legumes...from the hospital head east on bodenackerstrasse...turn right on Astershagstrasse and head south, park at the end, and walk south for a few min...there are signs. they also have a corn labyrith, and some cows always nearby. its a nice fall outting.