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.
If you buy the item yourself, be aware that MIT does not reimburse for sales tax. You will want to give the vendor MIT's tax id number, which you can get from the treasurer and give to the vendor.