You will have hard time finding original cushions with the Grady Logo. You could have cushions made but they probably won't have the logo. You may find that new seats with cushions will be cost effective over custom made cushions.
Here is a really inexpensive option
West Marine
www.westmarine.com
You will need to reuse your slide mounts or buy new ones (probably a better idea since I would bet you old ones are pretty worn out