Dear Lucy,
There is some more info I have for you about different products and companies(I just got back yesterday from a vacation at the beach and my mind must still be at the beach!) which I neglected to add to my previous answers to your question.There is a company called ABAIP which sells various disposable briefs.I haven't ordered anything from them but the company has a good rep.It's run by a person who has incontinence.The web address is www.abaip.com and the phone number is (888)401-5311.
The website for Euron briefs is www.euron-info.com. I don't know if they have a U.S. distributor for their products.According to their website they've been around since 1979.I emailed them to find out if they have U.S. distributors.I'll let you know what they say.
There are prouducts you can add to disposable briefs in order to make them more absorbent.They're called diaper doublers.They're also known as booster pads or liners.They're made specifically for disposable briefs which are tape-tab diapers.I don't know if they make them for pull-ups.Apparently Toys R Us/Babies Are Us sell diaper doublers.They're made to fit into baby diapers so I don't know if they'll fit into youth briefs.
I hope this info helps and if I come across anything else I'll be sure to pass it along.
Colin