Surplus mil wool trousers are always a good option..

Here's two pair for less than 20.00(German) w/o cargo pockets that'll give good service for med weight wool trouser needs.

http://www.sportsmansguide.com/cb/cb.asp?a=151230

Goodwill, Value Village and surplus stores have decent wool trousers for pennies on the dollar of what you'd pay new.

Wool dress pants from the thrift clothing stores , layered if necessary, can be found at thrift stores with suits..Jim