I don't know if they are interchangeable with the Class L or M, but honestly have not attempted to try anything other than the recommended style. I order mine online. These are the ones I purchased. At that time I bought them, they had the best price/lowest shipping:
http://www.thesewingoutlet.com/searc...5623AQ&x=0&y=0
I did purchase some on ebay once, but returned them. They didn't have a hole to pull the thread thru to start the winding and wouldn't wind properly. Be sure what you buy has the holes.