Ma's Cycles, a Triumph dealer in North Dakota, offers a turn signal kit for the large and small silver bullets. But as you can see if you follow the link, they're made for the existing mounting holes in the license plate housing. I'm sure yould adapt them to where you want to put them, but consider this....with a passenger on the back, their legs will block the view of the turn signals if you mount them on the most forward bolt hole of the fender rails.
My wife rides with me a lot, and I went ahead and bought the kit a couple of years ago and just put them where the old ones were. I took this pic just the other day, if you want to see what they look like on the bike.
