I believe using "of" is correct when using "myriad" as a noun, the same as "many" or "number" (the noun form of numerous). "He had a great number of seashells", "she possessed a myriad of skills".
Apparently it used to/still means 10,000 so it should be usable anywhere 10,000 is. "There were a myriad of them"/"there were 10,000 of them".
O that’s funny. Apparently it was originally a noun, hundreds of years ago. It actually changed into the adjectival use I was referring to earlier in the 1800s.
If anything it seems that using myriad as an adjective was actually an example of a rule change made to accommodate how people were speaking at the time.
Apparently it used to/still means 10,000 so it should be usable anywhere 10,000 is. "There were a myriad of them"/"there were 10,000 of them".