has different sizes of tags - their small tags are nice for cats, and they have a tiny oval tag suitable for kittens or ferrets. I've ordered lots of tags from them over the years, and have always been happy with their service. www.tagexpress.com
has some cute ones too, though I haven't ordered from them yet.
Just a caution: if you want to put collars on your kittens (this applies to full-grown cats too), it's important to make sure they're safety collars that will break away or stretch all over in case one of them gets caught on something. Since cats & kittens jump up high on things & get into all sorts of tight spots, it's quite common for them to get their collars caught up on things. If that happens, the collar needs to be of the type that will allow the cat to break free, to avoid a potential tragedy.Beastie Bands
are good allover stretchy safe collars that will even fit small kittens. I like Safe Cat breakaway collars too, but the latch is probably too strong for a small kitten to break open (put one around your wrist & try to break the latch with a force equivalent to that you think your kitten or cat can exert to choose the right one for your cat). I don't like the ones with just a little section of elastic for the safety release, because my cats have gotten those caught in their mouths, and I've also come home to find a cat with his leg stuck through one.
You can find safety collars at pet shops, and you can also probably find several good choices in the Cat Shop
section of this site or on Meowhoo.com.
Hope this helps.