Unfortunately if you want to get shoes as expensive and coveted as Christian Louboutin then you'll probably have to pay top dollar. There are plenty of fakes out there and it's probably not worth risking the wrath of your girlfriend when she finds out they're not authentic!
With high-end women's fashion such as handbags and shoes there aren't really that many money-saving resources, and usually these luxuy commodities are rather pricey. However, always keep your eye out for a bargain and try net-a-porter.com which sometimes has a great sale on authentic Christian Louboutin shoes, but don't hold your breath!
Sorry I couldn't be of more help, I think you're going to have to bite the bullet and dig deep into your pocket for her!