Nowadays, caskets are available for rental, just to fulfill that need. Speak with your Funeral Director about your preference to rent, rather than buy, a casket for the viewing and they will be able to show you the options available for rental.
There are also caskets specifically designed for cremation – usually they are made of a fiberboard material, and most look very much like the more “traditional” hardwood caskets. Again, your Funeral Director will be able to advise you of your options.