As Shrekster says look for a breaker that has a header section or Exup valve, in the UK there are some part-search utilities that charge you a couple of quid but search a good number of breakers catalogues for what you're looking for. If you do buy a header then explain that you are only really interested in the exup valve and assembly so ask for pictures of this in and out of the header so you know you're not buying another seized one. Take your time and something decent should come up. Obviously check Ebay, I had a look but could only see ones in the states one was really grotty and the other was overpriced as it included an end can.
The valve hasn't been working since you've had the bike so you could just refit it and be in no worse situation whilst you look for a replacement. I'd not bother refitting the cables though and I'd make sure the servo is still operating by checking that the cables are moving when the ignition is turned on, the servo will have had a hard time trying to operate with a seized valve. What position is the valve stuck in, is it against the stops or somewhere inbetween? Taking a hacksaw to the valve and removing it from the equation is an option just so you know you're getting full power but if you're used to the bike like it is then maybe don't bother.
The valve hasn't been working since you've had the bike so you could just refit it and be in no worse situation whilst you look for a replacement. I'd not bother refitting the cables though and I'd make sure the servo is still operating by checking that the cables are moving when the ignition is turned on, the servo will have had a hard time trying to operate with a seized valve. What position is the valve stuck in, is it against the stops or somewhere inbetween? Taking a hacksaw to the valve and removing it from the equation is an option just so you know you're getting full power but if you're used to the bike like it is then maybe don't bother.