I am not familiar with Rosco gels. As for the 'really dark' wratten filter, there are several, and it depends on which film you are using as to which filter you can use. Each film and filter is sensitive to a particular part of the IR spectrum.
Mark,
A couple of the common IR filters are #'s 87/87c/88a/89b. I don't know for sure but doubt Rosco makes these #'s. They were available in Kodak gels. If Kodak's still in the gel business they're not real cheap though.
I've picked up a couple at camera flea markets & on closeout tables at camera shops.