Check with your farmers market, I'm not allowed to do any form of 'food prep' at mine, everything has to be sale ready when I arrive, not to mention refrigerated frosting is hard to pipe with :)
I get dry ice for pennies at a local place, and use that to keep everything cool. I don't take cupcakes with perishable frosting/filling to the market though. I also don't live in an area that gets too much heat, so I've never needed to worry about going as far as buying a unit.
I know this is a month old, but I thought I'd share what I use. One, I don't do any food prep at market, and I wouldn't think that would be allowed for sanitary reasons. However, I do bring a small refrigerator for mini key lime pies, cheesecake slices, and cupcakes with cream cheese frosting.
It had to be small enough to be manageable for transport every week, but almost all the small fridges I found didn't actually seem to have a real condenser like a large fridge. The dorm and beverage size ones simply kept things about 30 degrees cooler than the ambient temperature, which doesn't work for an outdoor market in the summer.
I ended up buying a Danby 1.8 cu ft mini refrigerator (it has great reviews), and it has worked wonderfully for me for a year and a half now. It stays COLD, and it doesn't have an in-the-way "freezer" compartment. It is definitely on the smaller side, and I've started packing it pretty full on market days, but it wouldn't be very manageable for me if it was much bigger.
I hope that helps!
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