My coworker bought a personal one and she says she loves it when pairing it off with her 13.3" when we do audits. We do accounting so a lot of inputting of numbers too. She paid 50 bucks (CAD) for a wireless num pad and says it was a hard sell price wise when she first bought it, but after a good 3 years of hardcore use, it's worth every penny.
No idea where she bought it though. I believe it was in Hong Kong.
There are ones with cords. Some have a short cord, so a mere purchase of a USB cord extender may be in order. However, the mobile feel of these aren't as awesome as the wifi one. Being able to move your num pad closer to a book or whatever you need to input rather than be "restricted" to the laptop is really nice.
Looks like there's a few on newegg. Quick glance shows price range of 10-25 bucks for a wired one. No wifi ones I currently see.
NCIX looks better for these...
http://ncix.com/products/?sku=19725L&vpn=AKP11CA&manufacture=TARGUSJust keep in mind to have some back up battery power or whatever is needed for it.
IMO, there was a wired Lenovo one as well that looked super comfy to use. It was 55 bucks though. However that style of keyboard/keypad is a personal fave and I don't see it on any other product. The above is about 35 minus shipping. I'd personally budget about 50 bucks for one. I've had VERY good experiences with lenovo products before, so I wouldn't personally put it past me to pay the 55 if i needed one super badly for constant inputting. (I don't at the moment, since I have access to a wired one already) Targus isn't too bad though. They're pretty decent.
TBH, wireless is far more comfy to maneuver than USB ones. Same idea as a corded vs wireless mouse. On rare occasions, the wire is really annoying.
Yet another note, consider buying more than 1 wifi one? Then you have back up AND, if you find that your works super great, you can probably sell it to another student for profit. A sorry sap who didn't buy a comp with a num pad.

Search term: USB numeric keypad