Many supplies SIPB uses are ordered using requisitions. What you do is get the req book (talk to the Treasurer for the current location), take a look at some of the older filled reqs and imitate how it was done.
For a req less than $200, get an officer to sign it. For a req more than $200, get an officer to initial the req and send mail to Jane saying that you are going to go over there to get them to sign it (e40-321; Jane Trull, janej@mit, e40-315). After you have gone there, you can spend the money.