For the garments, look at SanMar and Charles River Apparel.
They seem to have a good selection of youth to adult matching items.
I've just done a job for a muscle gym with vinyl down the left leg. I did some hunting for placement ideas. (all are down the left leg or across the b'hind)
Adult sizes, 6" down from the waist, centered about 5" from outside edge of pant seam. Lettering starts at top. I also did the same with the lettering stacked. Try on the pants and lay a piece of tape down the front of the thigh. The placement won't be where you think it should be until you try it on.
I saw a few samples of putting stacked letters on the outside of left leg, just in front of side seam. Centered top to bottom.
I found one sample of large letters on side of pants, but it read upside down when the person was sitting. I'm not a fan of that one.
I've also put lettering down the left sleeve. Read this for more ideas.
Hoodie sleeve placement?