Having been unable to find any information on Iowa's Farmers Markets beyond a simple listing by city, date, etc., I did some data analysis of these Markets myself.
First, using a blank map of Iowa, I created a green circle to represent each of the 174 Farmers Markets in Iowa. That map is attached to this post under the name "Farmers Market Map." These circles do not represent the sizes of the markets (information which I could not find), nor do they point to particular cities--only the counties they are in.
Next, I found the population of each Iowa county (US census estimates for 2003 populations were the best available data). I entered these numbers into a Microsoft Excel file, along with the number of Farmers Markets in each county. I then divided the number of Farmers Markets in each county by the population of that county, and multiplied the result by 100000, to get a number that is easier to work with (for example, 12.623 instead of 0.00012623). This number--in column D in the spreadsheet 'Sheet 1'--represents the number of Farmers Markets in each Iowa county per 100,000 residents of that county. A higher number indicates a greater number of Markets per capita, while a '0' indicates that the county has no Farmers Markets. A graph of this information is available in 'Chart 1' in the same document.