Hello fellow Kentuckian!
If you live near Bardstown I could make you some! :) I make clothes for my daughters when we go to Disney, so I understand you wanting to do this for your grandsons. I'd recommend you look into two options.
The first would be looking on
www.disneyshopping.com at their 'Disney Design' section. There are many design-your-own options there, and you could order a shirt personalized with their favorite characters as well.
The second would be to seek out a local seamstress (or look on eBay!) to make you something. You could, again, have created anything you can imagine. An advantage to this option is that you could request literally anything and not be limited to the stock styles/sizes/characters on the Disney Shopping site. Most designers on eBay will custom embellish major brands of shirts, so you can select that way to ensure a proper fit. I know with my girls that's hard to do when ordering from an unknown website - I've found Disney's sizes to be very inconsistent.
I hope you are able to get just the right shirts for your grandsons, and have a wonderful vacation!