See what you find at target, but if you're not happy and decide to look again for tubigrip -

Tubigrip size J/K/L/M would normally fit trunks/bellies from the tiniest size 8 to... well, I'm not sure, but M is pretty big - maybe a size 20? A double layer is tighter than a single layer, so that gives you the equivalent of half sizes, kind of. Chemists don't usually sell it, because these sizes are only packaged on a roll, and you need to cut off the length you want. The chemists normally have the precut, prepackaged sizes which are graded for limbs. You could try calling any local women's health physiotherapists, and see if they will sell it cut to length, without you being a patient. Or call your GP, or CHN, and see if they have it in those larger sizes.

Having said all that, I plan to use my pregnancy belly bands, which I've made from ribbing from the local fabric shop, and just sew the circle a little tighter as I need it!