This thread (
http://greekchat.com/gcforums/showth...hlight=letters ) has a link for the patterns of all the Greek letters. The Delta is not a equilateral triangle so that may be a problem for the OP.
I have seen professional team logo fabric at the chain fabric stores such as JoAnn's and the remaining Hancock's. Usually the teams reflect the area teams. If you live in the Boston area and you strike out at JoAnn's, try Fabric Place. The Framingham store is their largest location.
Prewash and heat dry your fabric and clothing item to remove the sizing. To iron the letters to the shirt, I would recommend Heat and Bond for all fusible applique as opposed to WonderUnder. If you sew your letters on a sweatshirt, you will not need any stabilizer. For t-shirts, you should use tearaway stabilizer or even freezer paper. I would recommend satin-stitching over a zigzag stitch. If you decide to zigzag, make sure you seal the raw edges with FrayCheck.