Generally, (At least in my chapter - Iota Rho) a new member's first set of letters are given to her by her big sister. However, Once she's been initiated, she will want to wear her letters everywhere, so getting her another lettered sweatshirt would definitely be a good idea. Here are some websites that you can get them from:
http://somethinggreek.com/shop/
http://www.designergreek.com/
Another idea for you: a lavaliere is a gorgeous piece of jewelry for you to give her. I got mine from this website:
http://thecollegiatestandard.com/col...lpha-chi-omega
Hope this helps!