I've seen alot of used turn signals on Ebay, off of the later model Spirits, that will mount on the fork tubes. I considered doing the same thing as you,( ie: moving them down to the fork tubes) The only thing that would have to be changed is to extend the wires. >PD<
Like PainterD said, I think your best best is to use signals from one of the years of vt1100's that used fork mounted signals. I think that would be '97 on up for the unnamed/spirit models and '95 to '98 for the ACE's (assuming the ACE has the same diameter forks) Look at bone yards and ebay.
I used kuryakyn fork clamps - they're about $35 and they make them in 41mm (I think that's what the 1100's use) but I don't think they'd pair up with the stock signals.