My cat t-shirts are quite popular and when I am out wearing one I always get someone asking where I purchased it, or if I could make their cat into a shirt. Each one is individually made and prepared. This is for a couple of reasons. I have quite a few graphics and people take different sizes. The up-front costs of producing t-shirts in bulk and then storing inventory is not desirable to me. This means the individual cost is a bit higher than you would normally pay for a t-shirt if you were to drop into a graphic t outlet that has an in-house workshop, or a store that orders in bulk. Since they aren’t made in bulk you will have a truly unique shirt.
When you order a shirt, I use a place that prints vinyl (for the black and white) and I have them print the graphic on the shirt. The black shirts are best because the black fabric serves as “negative” space in the image. The pop-art styles are printed in colour so there is a bit more variety in colour.
To answer the other question, if you send me a high-quality image of your cat in a pose that would look good on a t-shirt I can create the graphic. Imagine the cat as a silhouette from the photo you like. Or alternatively, I can add your cat to the more colourful pop art style background.