I found Farnell's search interface obnoxious (same with Digikey actually; gate-source breakdown voltage isn't a searchable parameter), but I did find one device:
ST Microelectronics STQ3N45K3-AP
ID,max = 600mA
VDS,max = 450V
VGS,max = 30V
With respect to the above gate-source breakdown voltage, it's also self-protected via a back-to-back zener diode! Neat-o!
It's roughly $0.50 - $0.60 on Digikey, $0.91 (converted) on Farnell/Newark, and non-stock at Mouser.
WITH ANY REPLACEMENT TRANSISTOR, SINCE YOU'RE USING THE BOARDS THAT GO WITH THAT SCHEMATIC, WATCH FOR THE PIN-OUT. I CAN TELL YOU FOR CERTAIN THAT THIS PART DOES NOT MATCH THE PINOUT OF THE 2N7000
Personally, again, I think you can probably safely protect the 2N7000 with your own zener.
EDIT: also be aware that the threshold voltage is moderately high compared to the average. It's about 4.5V, which still shouldn't be an issue. It's just impressively high, in my opinion, for a tiny TO-92 device.