Straight or pull back risers? Drag or pullback handlebars? Either way, you can get risers for Harley Davidson bikes that will work. Just buy ones that don't have a connected top cap, the spacing is wrong, you would have to drill new mounting holes in the top fork clamp. With non-bridged  HD risers, at most, you might have to redrill the factory Suzuki riser mount holes to 13mm diameter.

Other good choices are take off parts from Yamaha or Honda v twin cruisers. Older Suzuki Maruaders had decent risers, as did Kawasaki VN800s. Surely there are bike breakers in the UK with websites...