Hello Andy,
The sensor NTC103AT2 should do the trick. Available (in Holland) at Conrad Electronics. I think you cann buy there too using Creditcard.
A complete new one, you can stil buy at:
http://www.scientificsales.co.nz/la-crosse-ws7000-25-outdoor-thermo-humidity-transmitter.html
Price is in NZ Dollars.
Goodluck