My girl wears size 12 mo and is out of diapers. I can alter the 2T if I need to (and I do know those swim on her because my 3.5 year old wears that size), but I'd rather just buy a pack if there is such a thing. A quick google search yeilded nothing.
Who makes undies smaller than a 2T?
I started a thread in this forum just a couple weeks ago asking the same thing. The only suggestion I got that actually seemed to have smaller than 2T was Hannah Anderson (sorry, no link--I'm sure you can Google it). The poster said she'd had really good luck with them, but I decided they were just too pricey for our budget. I got 2Ts for DS, and although they won't actually contain any #2 accidents (major leg gaps), they will stay on DS's waist.
Target sells toddler underwear (sold in the baby aisle, not the kid underwear).
My daughter also wears greendog underwear I got at Macy's. I think it's still size 2T or something, but the weight ranges are much lower.
And yeah, like the PP, it's usually 100% cotton, so it will shrink in the wash.
Do you have any of the diaper covers that came with dresses? I found that you can put the way smaller sizes, like 3-6 month or 6-9 month on a small toddler to use as underwear. When my son was right around 2 years, I put some really cute plaid pants on him that were 3-6 month size from Gymboree. They made adorable long shorts on him. All those baby clothes are made for diapers, so eliminate the diaper and you go can way down in sizes.
I got these and they fit my very skinny little boy at 15 months or so. They are Under the Nile tiny undies at the EC store.
I bought some 4t low rider undies and then I just took them in at the sides. They were low enough through the rise and when I took them in at the side it really shrunk down the leg holes. The ones I have are from Sears. I also made some and bought a couple of more expensive brands. I love the fit of petit lieu and my absolute favorite, though way WAY too expensive are made by katvig. But they're like 10$ a pair, but have held up the best of all dd's underwear and were the smallest that I could find. Gap/oshkosh are not that bad, they are big but not as bad as most other brands (old navy I found to be much larger than gap). I also know that H&M carries underwear that's marked as 18months, but I found those were larger than gap out of the package so I never bought them, but they might shrink a lot.
Good luck! Dd was only 18lbs when she pl'd so I know the pain of finding tiny underwear in a world where many stores don't carry underwear below a 4t.
My kids all potty trained before 2, so this was an issue for us, too. I also found that it was a bigger problem with girl undies than boy undies--that this elastic at the top seemed to help keep them up, somehow.
We got Gerber training undies (they used to be available everywhere, but when I bought more a year ago, I had to order them online). I bought the plain white ones and hand-dyed them into a rainbow of colors, and they're super-cute (I think). Plus they're more absorbent, in case of "close calls" or whatever.
The other place we've had luck is on Etsy--there's a seller who makes really cute boxer brief-style undies for boys and girls, and we've bought a lot from her. Here's a link to her shop.
Ds1 was a tiny 15mo when we moved into unders and the Gerber 100% cotton underwear (not the trainers with a plastic outer shell) fit great and really lasted! Hw wore them 1year + and they are still in great shape.
We bought them at JCPenny's and also found their Wee Essentials line to fit his slim build pretty well.
Gerber training pants, which are basically just thick underwear, come in 18-month size. They're inexpensive, which is nice. My 3.5 year old is still wearing hers, but we got them before she was a year old, and they fit her then, too. She also fit into 2T underpants then, and she's skinny. Some brands might run smaller or stretchier in the 2T size, so I wouldn't rule it out if I were you.