You can find TONS of cute designs - both customizable T-Shirts and decals for purchase on Etsy, and here on the DIS, there is a forum of creative DIS-igners who have a large selection that out of their generosity of time and talents, will customize for free. Additionally, there's lots of cute clip art available on the web to make your own design.
If you decide to make your own shirts (which it sounds like you want to do), I found "professional" quality transfer paper on
Amazon and it works SO much better than the Michael's/Walmart/regular store brands do. Search for "jet opaque heat transfer paper."
A couple of pointers: 1) Use colored shirts instead of white. I just made white shirts for the first time for our cruise in May - never again! White is just so unforgiving and it's really hard to make the colors as vibrant as you would want. The product I told you to search for is only for colored shirts.
2) Remember that on colored shirts, any area of the design that looks blank will actually be white. The good side to that is that if you want something to actually be white - it will!
3) You have to press down REALLY hard and use a flat iron (one that does not have steam holes) for the best adherence.
Way more info than you asked, but hopefully it's helpful!