Since starting to carry, I have always purchased pants with a +1 size, a suitable cut to the pants to allow the range of movement that IWB demands above and beyond just walking/sitting, and generally with a suitably stretchy material (ie, 1-2% spandex along with the wool/cotton/poly blend) to allow for no binding. Has worked fairly well, for me.
Though, the downside if a stretchier material and a somewhat looser waistband is the need for a more-solid belt. But it affords the extra room and range of motion I've found that IWB required. Pros/cons.
Example: Haggar
Cool 18 slacks -- polyester/stretch blend material; a mildly-roomier (not relaxed) cut around hips/thighs; and a stretch waistband. There are various type of wool/stretch blend fabrics out there, as well.