Once the traditional way of selling plants before garden centres started, you can still find a few traders at weekly markets.

Plant quality can vary but look for regular traders and if you get to know them they can be very helpful.
You might find some bargains, local nurseries that supply the supermarkets often use markets and car boots to clear surplus stock and some clearance stock can be found.

Local suppliers usually selling their own stock.
Some times surplus stock is sold at bargain prices.
Limited range of stock.

You may have to get the car out and make a special trip.
Most have too many non-gardening distractions.
You need good weather or an umbrella to enjoy your shopping