I've been experimenting with soldering white metal recently and have found Chronos Ltd to stock everything that's needed and are fast on deliveries too. Here's a link to their solder supplies page -
Chronos solder page.
You really should use the correct flux for the 70 degree solder, as white metal oxidises very quickly and the flux is designed to cut through that.
I also invested in a temperature controlled soldering iron which has proved to be invaluable. I tried the normal iron first, just holding it on the white metal for the briefest period, and trying to remove it before it melted the white metal, but it's a very hit-and-miss technique and not to be recommended.
It's really straightforward using the temperature controlled iron, the correct solder and the right flux