a number of CS7700 cassettes are currently listed on eBay - including a few new
but don't know if they are the exact cassette you are looking for
a 9 spd Ultegra cassette should also work ... possibly 9 spd XT cassette also
there are some other Shimano compatible replacements that I'm not familiar with ... ? ... Sunrace ? ... Miche ? ...
don't forget to also get a new chain