Farmers markets aren't new by any stretch of the imagination, but they have been growing in popularity in the US and Europe as people have become aware of the benefits of buying local. However, in Japan, farmers markets can often offer a fair amount of savings by cutting out the supermarket middleman.
Despite the one stop convience of you local mega-mart, it is not always the cheapest source of fruits and veggies. Instead, look for a 農作物直売店 (nosakubutu chokubai-ten), or simply chokubai-ten (直売店), which literally means a direct seller of agriculture goods. These are usually small, community run stores that sell veggies, meats, and just about every other agricultural product that your local old folks decide to grow. For the most part, these shops are permanent storefronts as opposed to open air markets that pop up every weekend. If you can read Japanese, check out tyokubaisyo.com for a prefecture by prefecture list of chokubai-ten
|The chokubai-ten near my house|
Also, if you are in need of omiyage (お土産 or souvenirs), the local chokubai-ten is a great place to get your hands on stuff made right in your own back yard. Since bringing hometown specialties to whomever you visit is consider polite, your local chokubai-ten is a good, cheap alternative to the highway rest stop gift shop.
The only down side is that you are, for the most part, subject to the whims of what your local JA (the Japan Agriculture Cooperative) and various retiree-cum-farmers want to grow. Also, if something is out of season then expect to pay more or not find it at all. Also, keep in mind the veggies are very, very fresh. Like, still has dirt on it fresh. So you might have to do a little bit of cleaning before eating.
In more urban areas, more and more weekly or monthly farmers markets have been popping up. One example is the JA Tokyo Central Farmers Market . Other municipalities and civic groups have been trying to promote cheap and healthy locally grown foods by planning their own market events.
There is no nationwide registry of farmers markets online (that we know of). The best way to find these events are to ask around town and keep your eyes open for any big gatherings. You can also look at your local community bulletin board board, where you might come across fliers advertising a ファーマーズマーケット, an 朝市場 (asa ichiba), or something similar. More popular events might have an online listing; for example, a Google search for "広島 朝市" turned up a farmer's market held every Sunday right in downtown Hiroshima.
Although not as convenient as a year-round storefront, these markets usually repeat on a regular basis (weekly, monthly, annually, etc.) If you find an event you like, mark your calendar and be ready to go next time it comes up.