With so many different species of trapdoor spider it'd be hard to give a general habitat preference. the most useful information I've found when it comes to creatures is either from other hobbyists in my area or from species descriptions. neither is perfect, some hobbyists dont like to share that kind of knowledge and a lot of species descriptions can be vague or leave those details out. finding out the habitat preference will help you in your quest. I've been searching for Sphodros in my area in their known habitat preference for the better part of 2 decades and still nothing. one day I'll find one in the wild, but so far its elluded me in a big way. good luck!
what i would do is look in platnick's reference catalog http://research.amnh.org/oonopidae/catalog/
and look for any pdfs on there for each of the species/genera listed in my TN link. if you are reasonably lucky you can find papers that have lists of materials collected. that will be a list containing coords for all the spiders collected in the paper. though i have never actually used such a method you can probably see the benefit. i do have a few coords pretty close to me, i just have never gotten around to going to them
and i got to say... some ppl have the gift and others do not. i... do not. i looked in the same area for almost five years (dozens of times in those years) before i started finding mygs there. and so far i have found dozens of specimens representing at least three different species in two different families.
sometimes we find true trapdoors by feel or by luck. by feel means actually feeling the ground with your fingers. by luck means intersecting a burrow while going after something else
in your state the Myrmekiaphila and Ummidia are the ones that make true trapdoors (i think. ummidia for sure does, and the myms either make wafer or folding type doors... i think)
one trick way to find these guys is to look on eroding river banks and road/trail cuts into hills. this can expose burrows. if nothing else, once you see some old burrows you know you are probably in the right area... and sometimes you can actually find spiders in minorly exposed burrows
i would love to be able to do a write up about finding traps... but i really do suck at it. the only two true traps i found the area i mentioned above i found by literally intersecting their burrow on accident. the second i kind of tripped and put my hand on the ground.. and poked my thumb into the spider's burrow. the first i flipped a rock that hooked another rock and ripped the spider out of it's burrow and i got it on tape
oh boy, i just noticed you have Sphodros rufipes in your area. this is a purseweb (smaller sized myg with disproportionately large chelicerae and fangs) in which the mature males take on bright red legs! it is locally and state protected in some areas, but not apparently *federally* protected. i am really interested in this species and so i will try to do some googling for you. it is annoying how incomplete my range of searching is, though. here is a pic of a MM http://upload.wikimedia.org/wikipedia/commons/5/50/Sphodros_rufipes.JPG
we can talk about other ways of finding areas with mygs in them, like pit traps, too.
I once found some sort of myg in WV when I was a kid. It had a dark body woth bright red legs. I was doing some garden work for the stepmom, moving dirt from a big pile into some rows. The myg just came crawling out of the middle of that dirt pile as I was shoveling so sorry, I can't tell you much about habitat. It scared the bejeezus out of me at the time as I was only 9 or 10 and had never seen anything like it. I figured if it was that red it must be deadly. That and the first time I found a wild Latrodectus really stick in my mind when I try to figure just why I like spiders so much as an adult.
It's weird, because I was very scared of both of those incidents.