I have found lots of seamless panties that are not boy shorts, but I'm afraid it's the confluence of the two attributes that I'm after. I have not found anything that is neither a boy short nor a thong (of which I'm just not a fan) that doesn't show a panty line, and the extra leg length of boy shorts also serves to prevent most of my thigh chafing. When boy shorts are not seamless they always seem to have the center seam from front to back waist, which I have found to be basically a torture device.
Unfortunately my budget does not allow me to shell out for the Soma ones, but it looks from the pictures like they do have the center seam anyway.
If anyone has anything to suggest that is not a boy short but nevertheless does not show a panty line, I can probably find some other way to get rid of my thigh chafing; but Lane Bryant doesn't have anything, and although some styles turn up on a Bare Necessities search, they are either shapewear (which means they wind up costing at least three times what I can afford) or they say they go up to a 9 or 2X but they actually stop at an 8. I'm beginning to think I will have to try to knit my own, which promises to be a failful endeavor.