32-36 hole 700c rear wheel try amazon.
there are bike shops on the web where you can probably buy a 36 hole wheel for your bike.post in other sections of this site also about buying a new wheel.but before you do that ride it and see what happens,if it happens again the wheels cant support your weight.
or go to a bike shop that sells bikes second and repairs bikes first.one of those shops where there is an old guy smoking his cigar.Dont buy from a trek dealer.
here is a source
http://www.niagaracycle.com/index.php?cPath=135_200
Dont know your size or if they even have your size or gears on your casette.
Dont worry if you buy the new wheel and have no problems and ride the bike like there is no tommorrow you will have lost 50 lbs and will put the original wheel back on with no problems.and you have an extra wheel if you didnt wear it out.