When Riga Central market is closed (check the Central Market article) life is not over out there. At the back of the central market every night farmers from all over Latvia come to sell vegetables, fruits and berries directly from their minibuses. This is my favorite night vegetable supermarket. I like it because it is very convenient and I can go there after work, even when regular shops are already closed.
I like seasonal and local food, that is one more reason. Here you can buy directly from the farm. Everything is local (mostly!). There are some wholesalers and Lithuanians, but they are a minority.
The market runs all night, but after 23:00 some farmers might already leave. But there are still quite many left. I haven’t yet been at 04:00 and 05:00, but my friends say that around that time there are still many farmers and the prices are better, because wholesalers come to buy vegetables to resell them at daytime. But at any time you can ask for a discount if you buy a bigger amount.
Do not forget cash, and take some shopping bags with you. Save the resources ;).