Rentals in Ithaca are plentiful, they tend to follow the school year with leases starting in June, July and August.
Having said that...short term rentals are diffucult to find in this area.
I would suggest starting your venture searching on Craigslist, Housing Solutions and the Ithaca Journal or Ithaca Times sites, or the Cornell University Off site housing list.
There are also a few companies specializing in rentals:
Ithaca Rental Housing Contact List:
Horizons 607-257-6059
Housing Solutions 607-272-6091
Service Connections 607-277-1929
Certified Properties 607-273-1669
Ithaca Renting Company 607-272-3000
CSP 607-277-6961
Ithaca Rentals & Renovations 607-273-1654
Integrated Properties 607-257-5200
S.B. Ashley Management Corp. s 607-257-1725
Professional Rentals 607-257-0383
Northeast Short term Rentals
Kimball Real Estate 607-257-0313
University Park Apartments 607-257-3300
West Shore Apartments 607-273-7368
Barrett Properties East Hill Duplexes/Townhouses 607-272-5296