mule spinning is almost a lost art --there are 2 yarn mills in US that still provide mule spun yarn.

Worsted yarn is strong, firmer, and the machine that spin it are easier to maintain.

i suspect if you google mule spun + yarn, you'd find the mills, one is in NE, the other in MI.

i am not all that into spinning. i know there are different ways to spin and ply yarns, (worsted, cable and mule spun being the most common--perhaps even the complete list!) but i really don't have a full understanging of the differences--at a spinning level.. i can recognize the differences in the yarns.

in the US mule spun yarn used to be called 'germantown'--and centeral PA was the sorce for many 'germantown' wools.
what every you call it, the yarn is softer, and has more loft--(but it also is more likely to pill.) germantown yarn was the traditional choice for infants cloths and blankets.