Several small affiliate programs feature creatives in this size, as do most CPA networks (ie Websponsors.com, DirectLeads, OnResponse, Safe-Audit, FineClicks, etc).
Amongst the major banner networks for small-medium sites that support buttons are: Advertising.com, BurstMedia, Zonfire and ValueClick.
Standard Internet's BulkClicks, ClickThruTraffic and AllClicks also support 120x60 graphical spots.