If you've just joined Theta, and you're still a new member, note that Theta is one of two NPC sororities that does not allow new members to wear letters. The other is my sorority. I believe there may be one or two other NPC sororities that don't allow anyone who isn't an initiated sister to wear letters, but Theta and AEPhi are the two of which I am certain.
That said, I ordered my first AEPhi letters while still a new member. I was in a local sorority that became a chapter of AEPhi, and there was nothing stopping me from
ordering the letter sweatshirt - I just couldn't
wear it until I was initiated. (And, believe me, I couldn't wait to change clothes post-initiation!
)
If you find a kite or pansy charm, go for it. If you find a Greek letter Theta charm, I'd suggest you go ahead and buy it, but not wear it until after initiation - but I will defer to actual Theta sisters on this point.