Aimee, I had a K&N fitted to my TT along with the exhaust I sold you.
As soon as it was fitted I noticed the difference, better pickup on the throttle right from the get go! the cheapest place I could find after a lot of hunting was:
http://www.ybfilters.co.uk/bikefilters/triumphtable.htm
£32.75, including Free Delivery!
Also got one for Rob (Sigg21) Sprint ST when placing the order, Fast Delivery
You could have had myn but its currently fitted to my Daytona being as they have much similar Air Boxes.
Hope This helps, And to all others take a look at 'YB Filters' for your new Filter!
Dave
