I go to Hobby Lobby and buy the Gold Grid non-woven (doesn't stretch) which is similar to Pellon's red grid at Joann's but a lot cheaper at HL if you have one - use a 40% off coupon as well. If you'd like to send me an e-mail, I've written up how to do T-shirt quilts into steps ..... have done many of these.
I am sorry to hear of your loss. This will be a difficult quilt to make, I know. My brother passed away last month and I'm hoping I can get thru making a couple of quilts from his clothing for his kids. It won't be easy but the long term is that his daughters/wife will have a bit of comfort from them. My heart goes out to you.
[email protected] for contact if you want the info.