By quana from Dayton, OH
This isn't for straw hats, but since you're wanting to decorate hats for a tea party, maybe this will be an option. My daughter and I made some, and decorated them with marigolds and lantana from the garden, fun & cute (and inexpensive!).
Try your local craft store.
Try www.orientaltrading.com. Search "straw hats". There's a couple that if you paint them, add fabric or a bow, can be real cute. Plus the price is great. Sold in bulk which is good when buying for a party.