If you don't know any men who are a plus-size, this time of year you can usually find that thrift shops sell off their Hawaiian shirts really cheaply. Also, there is always FreeCycle where you can get the shirts for free, just for asking. This way you don't have to waste a perfectly good shirt, just someone's castoff for your first sample.
Although I haven't made the cobbler's apron yet, it is in my future plans. However, I have made quite a number of aprons using men's shirts for our senior center to sell. They sell very well.