Just some things from licensed vendors linked from the Sigma Pi web site:
Sweetheart pin from Herff Jones or whatever the company / division name is:
http://www.hjgreek.com/index.cfm?eve...roduct_ID=8439
Sigma Pi Sweetheart hooded sweatshirt:
Item number 040-027780 in “sweatshirts” from Spirit Recognition
http://www.spirit.cc/sig_pi
(I'm not necessarily endorsing either of these ideas, but they do give an idea of what's out there.)