Booking two one ways will be significantly more expensive than a roundtrip ticket on one carrier or alliance.

If you want to travel on different Airlines (which can be also done on one ticket, as long as you stay in the same Alliance like outbound on Air NZ, back on United) try Kayak, Google flights, Expedia etc. A lot of them will show you Alliance wide tickets, often ticketed on a codeshare (i.E. it looks like a NZ ticket all the way but certain legs are operated by another carrier like UA)

If you want us to help you out it would help to specify the dates you are looking at and the budget you have.