I made 3 bowtucks bags. I really like that design. The pattern calls for Pellon 987 and I think it's great stuff. I bought mine through fabric.com. I found that they had the best price for the size you get. 40 x 60 The packages come in a couple of sizes so double check. I think Joanns package is 20 x 40. Of course, if you can get it off the bolt, then you get exactly what you need.
But the real secret to the structure of this bag, is to put a foundation in the bottom of the bag, so it looks square. That info is in the pattern.
Good Luck