It is very rare to see affordable housing listed w/ a Realtor. There are simply too many waiting lists for each town. It is your best bet to contact the towns for their lists, and their criteria to make sure you qualify.
You might also consider alternatives that are not affordable housing, and explore your options down that path.... more
There are different ways. You can get an appraisal by an appraiser. Or you can get a market analysis from a Realtor (using a realtor that is familiar in your area and with land it the ideal situation). The appraiser will cost you money and more than likely the realtor will do it for free.