Hi,
Gigundo Capacitors are widely available: http://www.sonicelectronix.com/cat_i6_-capacitors.html etc etc
You could use this kind of power FET board with 4 - IRF540 FETS ($13.50) and just parallel the channels for extra margin:
http://arduino-direct.com/sunshop/index.php?l=product_detail&p=60
And they are optically isolated, so you could mount this as close to the solenoid / capacitor combination as possible and keep the Arduino farther away...
DISCLAIMER: I mentioned stuff from my own Shop... But I like finding solutions