maternity clothing!

Sara • FTM • DD Nov. 24 but born Nov.19 • Happily Taken❤️
Where can I shop for maternity jeggings? I usually wear 3 in jeans but since my tummy is getting bigger none of my pants button up or zip up. And regular leggings are not comfortable due to the elasticity around the waste. And how do I shop for them? Do I still get a size small or a bigger size?