I can’t find a cite for “90%” (though, I’ve heard that number cited on talking head shows), but I did find this that gives a partial list of who pulled out.
It may be that those sponsors who pulled out contributed 90% of the revenue rather than being 90% of all sponsors. Even if the 90% number is inaccurate, it’s clear that he lost a number of bigtime sponsors.
I’m surprised none of his detractors published the names of the sponsors who did not pull out, so they could be boycotted. 
They probably did not have time. He was dropped within two days of the big guns (Procter & Gamble, American Express, Staples) pulling out and there were enough other sponsors making similar noises that publishing a list of sponsors to boycott would have backfired if it included sponsors who dropped him between the compilation of the list and its publication.