Hi Jaquelyn,
I got my Sigma lavaliere from our national headquarters (and they got it from Masters of Design). I'm sure you could find one in an online catalouge (but I'm not sure which ones are on the approved vendors list).
Aside from that, rcsportswear.com has cute Sigma stuff.
If you need any more info, keep posting.
Laura
Treasurer
Zeta Mu Chapter