75% of my work is commercial real estate but I've done quite a bit of residential in Bucks County over the last 12+ years. Nothing stops your agent from contacting some of the rental listings in the MLS to inquire about a month to month listing. Especially if they have been on the market for awhile.
When it comes to an investment property like a rental, every month the owner does NOT have a tenant, is a month they can NEVER get back as far as revenue. We do this in commercial "every now and again" (however a short term rental for us may be 1-2 years instead of 5-15+ years.
I've noticed that some residential agents don't seem to want to ask (or don't consider it). In addition to the great tip regarding Craigslist below (and of course the local paper), have your agent ask if a month to month may be possible.
In the eyes of an owner, a nice solid, well qualified, tenant for a few months is much, MUCH better than no tenant at all, for same time period.
If you can't find someone to ask, I'll ask for you....
Hope that helps.
Brian Luce ABR, SRS
Weidel Realtors - Doylestown
215-348-5600 x 163