I had a similar problem (broken impeller for an SL-45). I ended up replacing the whole instrument instead of using the replacement impeller, but it looked like it was the right one for the SL-45.
I got it from Gemeco Marine Accessories, 1141 South Ron McNair Blvd, Lake City, SC 29560. Tel: 843-210-7170. The fellow I spoke to at Gemeco was very helpful. I'm trying to add a picture of impeller I received from them - pardon me in advance if I screw it up. I'm a newbie at posting photos here.