That is one seriously awesome start, isn't it? I can't wait to see the rest of the Pineapple quilt!
When I shop - I ONLY take shirts that are labeled as 100% cotton. I start in the largest sizes and unless it's something really special, will only buy shirts that are XL or larger.
Check with your local shop(s) as to when they have sales. Each week, the local VoA has 50% off of a specific color tag. Plus, they have 50% off of everything a couple of times a year.
As for the cutting - check Bonnie Hunter's website for her "deboning a shirt" video.