I like Earth dog online for bright collars
http://www.earthdogonline.com/allproducts.htm
The pampered pup has cute stuff that's a bit fancier
http://www.thepamperedpup.com/
Also, I went to Michael's craft store and bought a plain white T-shirt that is made to fit a doll or stuffed animal and got fabric paint and painted my sorority's name on the back of it.
Here is a picture of my dog (pitbull/chi mix) in her Sorority T-shirt and skirt!
Also, if you are considering getting a dog check out
http://www.petfinder.com they have almost every breed of dog up for adoption and you can do a search by the breed, size, etc. I adopted Laney from there and I couldn't ask for a cuter/better dog!