Okay well I got Paige when she was 7 weeks. And the day I went to go and get all of her stuff I bought a collar, harness and a leash. Although I didn't introduce her to her leash until this weekend. I keep her collar with her tags on it all the time, but to be on the safe side I bought a cat collar, one of the one's with elastic in it I think they call them break away collars. Anyway just in case she starts to get hung up in it and I'm not around she can get out of it with no problem, but she got used to it really quick.
I found her an adjustable harness in the dog section but something I might suggest to you is that they do have cat harnesses and the will adjust to a smaller size that the dog harnesses so you might check that out.
I only put Paige's harness on her for a certain amount of time each day even if we don't go outside just so she will get used to it.
I hope I helped a little. Good luck with your puppy and I hope to see a bunch of new pics as she gets older.
~Angela and the group~
Oscar~ Male Crested Gecko
Burt~ Male Leopard Gecko
Ernie~ Male Leopard Gecko
Candy~ Female Ball Python
Gidget~ Female Leopard Gecko
Paige~ Female Cholcolate Chihuahua
Marley~ Female Harlequin Great Dane