How can I make much needed purchases without using checks from the SNT account for my sister who's disabled?

Yes talk to her bank and get you a debt card for your sister purchase and safe all of the recipet . Keep all tracking numbers.

I just wanted to say "Thanks" to PBRyter for the helpful information.
It is very much appreciated.

Quicken 2014 (and maybe some earlier versions) allows you to scan receipts, documents, etc. and store these relevant files with each transaction. Being paranoid and old school (and not good at backup my data files up), I also keep the originals. But IRS has accepted electronic receipts apparently since 1997 -- see http://www.irs.gov/pub/irs-tege/rp-97-22.pdf .

Thank you for asking this question and to the person who answered. It gave me something to think about.

Talk to the bank where the SNT funds are about getting a debit card. I am trustee for a friend with a Special Needs Trust and they said they couldn't issue a credit card but a debit card was OK. This allows me to make purchases and order supplies online. I print out the order summary when I place the order and when the item arrives staple it together with the shipping invoice. I keep all the receipts for the year together in a file so that if there is any question I can produce the receipt. I also use Quicken to track everything, so I can produce reports quickly. Note that Quicken is great for record keeping but does NOT take the place of receipts.