In my experience, Ranch Products makes the best moonclips. You can order through them in larger volumes or buy in smaller numbers (8 in a packet) through Midway USA.
You can also shoot the gun without moon clips. You only need them for simultaneous extraction. Without them, ya gotta poke the empties out with a stick.
Ruger sells the moon clips for the SP101. I've seen other brands at several companies online (not sure which stores though). Not sure about moon clips for the Speed/Service Six.