I also have a Fiskars 24 inch rotary ruler...I had to do a lot of Google searches over a year ago to find one. Fiskars no longer makes it. I finally found and ordered one at a source that is now out of them and also found one on Ebay. It works very well. But good luck finding one! They're not easy to track down and aren't cheap.
However, I also love cutting with the June Tailor Shape Cut. It doesn't include the rotary cutter, but I get good straight results with this as well. There are several options on sizes, and you can get them at JoAnns with a coupon.
Both these tools have helped reduced my wobbly cuts.