I bought a set of the alphabet and different stitching for check books several years ago at Five Star. It works beautifully . I have made many check book covers with it. I am not sure they are still in business.
Check out San Francisco Stitch Company.
If I recall they have had some at one time.
I have never made a checkbook cover, but I was wondering if you could not enlarge or shrink one of the designs you do have to make them fit.