Whistler has recently made available separately for purchase the rubber case that comes with their new TRX-1 handheld scanner (see attached picture). On their website it lists that it is compatible with their TRX-1, WS1080 and WS1088 which means that it will also fit the PRO-668 (as well as the PRO-18 and GRE PSR-800). It comes with 2 interchangeable colored inserts and the price is $29.99 (plus shipping): https://www.whistlergroup.com/usa/shop-online/scanner-accessories.html?p=1
The only other commercially-available case that I know of that is currently available for the PRO-668 is this nylon case for $12.95 (plus shipping) available from Scanner Master: PSR800/PRO18/WS1080 - GRE Nylon Case/Cut Out Back | Scanner Master (The similar leather case for the PRO-668 that is listed on the Scanner Master website has been out-of-stock for over a year so I assume that it is no longer available.)
The only other commercially-available case that I know of that is currently available for the PRO-668 is this nylon case for $12.95 (plus shipping) available from Scanner Master: PSR800/PRO18/WS1080 - GRE Nylon Case/Cut Out Back | Scanner Master (The similar leather case for the PRO-668 that is listed on the Scanner Master website has been out-of-stock for over a year so I assume that it is no longer available.)