Unless you can buy OE replacements from the manufacturer, TLS Guy is spot-on.
I had some 1986 vintage Hafler 200 speakers which had a tweeter fail. Being out of business there was little hope of getting the factory replacements. When I removed the tweeters I found they were well marked as being a Vifa model number, 6Ω. Lucky for me Parts Express had a few remaining, as Vifa had been bought out and they are still working for my niece.
If M-Audio can't provide the replacements your only hope is to remove the drivers and hope they're similarly marked.