How to create custom shirts with great quality

Hello I'm fairly new to the t-shirts business. I'm using a heat press and plastisol transfers to make my shirts. Some of the local competition also does the same process as myself but, the do custom t-shirts as well right on the spot. I know the use a printer(don't know what kind), heat press, and a cameo cutter. The thing is they can do these custom shirts on the spot and I want to be able to compete with them. I just can't figure out what type of heat transfers they use that can go on so great and still come out great after washing. Most of the custom plastisol transfer companies want you to order at least 100 of the same image and I don't want to order 100 of an image that I can't resell to another customer. Please can someone help me I have tried jet pro soft stretch and 3G opaque for black and white shirts. Jetpro soft stretch didn't come out good in the wash it ended up cracking but, the 3G opaque came out great. Again someone help me and tell me how I can create custom shirts? Thanks in advance..