Joyce,  There are ways to do this, but mostly you need to think it
through and do it safely.  Piano casters are _not_ to be found in your
local hardware store.  I would recommend contacting your Friendly
Neighborhood Piano Technician for some replacement casters.

It would be good to first remove one of yours, at least to get the
dimensions.  These are fairly standard, but there are variations:
three screws, four screws, etc., etc.  If you know some hunks with good
strong backs, they can pick up one end of your piano so you can place
some wood blocks under it.  Just remember that the piano bottom wood
isn't designed to hold up a piano except where the casters are mounted
-- so be careful!

You will find the casters are in a (usually cast) metal base that is
screwed to the piano.  Remove the screws and it should fall out.  Now
locate and order the replacement casters.  Once you replace the casters
on that end of the piano, you can do the other end.

A piano tilter is a great tool.  I have one that I purchased from
a piano technician who no longer wanted to do that kind of work.
I find I use it about twice a year, and for that it's worth it.
Mostly I bought it because I have to work on the bottom boards on two
of my own pianos.  One does need to be careful when using it, but it
dos make the job a lot easier.  I have seen plans for building one out
of plywood, too.

Good luck!
David Dewey
Northern California