You probably are very crafty. You probably are already well into the home made dinosaur costume by now, exceptional mother that you are. All you need to do is glue the marbles into the eye sockets. If however, you’re like me, you’ll be buying one, or in my case, two, Halloween costumes, and it is best not to do so at the last minute. I have found a great selection on line at:

Chasing Fireflies (both photos in this piece are from their catalogue)
Lilian Vernon’s Lily’s Kids
Pottery Barn Kids

Amazon
(the other day amazon was offering $10.00 off when you spend over $50.00)
Locally, there are a bunch of Halloween pop-up super stores, or it’s also great to support smaller local vendors like Landis Labyrinth, Jenny Becks, and Flicka and if you shop locally you can check out the costumes before buying them.
One tip:
Make sure your kids is POSITIVELY ABSULUTELY sure that they have decided on their costume choice before your buy. (Or at least check out the store’s exchange policy.) From day to day my son’s fantasies change, basketball player, clone trooper, musketeer, lion, pirate. I’m about to put a cut off date on his creativity, before taking out my Am Ex.
Also, Halloween costumes are pricey. If you have a friend or family member with older kids, you may want to see if they have an overstocked dress up trunk you can raid before you buy. On the other hand, a kid in a costume with a jack-o-lantern grin is pretty darn priceless