Can you explain why you're interested in vinyl in a sheet instead of the roll? It might be easier to answer you.
As with everything, there is better pricing with a larger quantity. But some places (Specialty Graphics Supply, for one) will sell you material cut down either lengthwise or widthwise.
I've only just begun pressing, but JPSS is highly regarded and is meant to be peeled off the shirt when it's hot. It might also help to gently stretch the shirt immediately after to help keep the transfer from going out of "square" because it is easy to "warp" the image because it does take a bit of effort to remove the backing.
Something like easyweed or thermoflex vinyls have a tacky plastic backing the keeps the design in tact until it's pressed. It peels off warm VERY easily.