headset for voip phone calls?

I have a 2007 macbook pro and I'd like to start using it to make voip phone calls with services like google call. I have an old headset that I used to use on my windows laptop, but I have discovered that the mic won't work on the MBP because apparently it is a line-in port, which only takes sound when the device has its own power.

I went to the Apple Store and asked a sales person what I needed to do this, but he had no idea and said they didn't sell any headsets.

I've seen some USB headsets in other stores, but they are quite pricey. Is there anything else I can use that will make my existing headset work on the MBP?

Thanks.