you might also try www.webairbrushes.com for Badger parts. Their website is a little harder to navigate, but you can find the parts there if you look a while, and from what I saw their shipping charges are fairly reasonable. Dixieart did appear to have the best pricing on the parts, but right now they're drop shipping everything from their suppliers, and the last time I looked at their website they were asking that you not place less than a 45 dollar minimum order from any one supplier (Badger, Paasche, Iwata, etc..) not a problem if you're ordering a whole airbrush, but for a couple of parts, you might be aware of that.
To expand on the post above, I'd be tempted to order the complete head/tip/aircap assembly, and the appropriate needle. I think I'd start with the medium tip / needle combo, let your sister see if she likes that.
50-0162 is the complete head assembly, 50-0402 is the matching needle that she'll need.
If you decide on the large, it's the 50-0163 head / 50-0403 needle.
Good luck!