HelloLanding - Specializes in furnished apartments for at least 30 days, not cheap but rates are often far better than local hotels for a full apartment (ex $60-70 a nite vs $100+)
VRBO - Part of the "OneKey" program that includes hotels.com (with shared loyalty program and cash back), often has lower prices than AirBnb for longer stays and sometimes has oddball rentals like campers, tent sites and docked boats
HomeStay - Alternative to AirBnB that focuses on rooms to rent / shared bathroom situations at lower prices than area hotels
Zillow - Easier to do a Google search of "zillow" + the city you're interested in and "furnished"
Rentler - More for traditional unfurnished apartments with long leases but has a convenient interface to sort low-to-high prices in an area
Local online classifieds from the big newspapers and the regional Facebook Marketplace are also usually good sources and have listings that aren't found anywhere else
Also consider realtors or apartment agents, who can help find furnished and shorter-term situations for free: