Where exactly do you live?
some ideas of free ways to meet other moms: local libraries often have story times; cool playgrounds often attract cool moms & kids; some hospitals offer new mom groups for free, and even though your son is a little older, maybe they can direct you to some appropriate groups. You could check out local La Leche League meetings if you're a nursing mama, or Babywearing Groups, if you're a babywearing mama. You'd have to google for ones in your area, or along the red line. Also, if there are any cool cafes in your area, cool maternity/toy stores, or places like that that have message boards, sometimes there are postings for moms groups.
I've moved cross-country, and had to re-establish myself a few times. I find that if I start doing things that I'm interested in, other people like me are probably doing similar things. Then you just have to be bold, reach out, and talk to people, let them know you're interested in a friendship/playgroup, etc. Sometimes things don't click, and that's okay, but there are likely other moms out there in similar situations who may be too shy to do the reaching out.
Danielle, mommy to Dylan 9.5.06 and Cooper 3.11.10 , living in Providence, RI