An insulin pump is small enough to fit in a pocket, or you can attach a clip and hang it on a waistband. There have been many times when I have picked up an item of clothing and thought, "Oh the waistband is too thin and flimsy" or "These pockets aren't deep enough." I even look for long shirts to hang over the pump. That's hard to come by when you have a 3 foot, 6 inch-tall three year old. Jeans and pants with zippers are much more practical to buy than sweatpants.
When Lainey wears a dress to church, I safety pin a pouch on the inside of her dress seam to put her pump in. The pump is too heavy to clip to her underwear waistband.
There's even custom made clothes for pump wearers. Take a look at this website and one mom's story.