Hi!
Which risers exactly are you referring to? I'm assuming you're talking about the risers which are 2,5cm, 5,5cm or 7cm high, respectively. Those are in fact standard risers used on the GS500E, depending on the model year. AFAIK the 2,5cm risers were only used on the 1989 model and are therefore very hard to get these days... since your bike is a 1991 model, I suppose you have the 5,5cm risers on it. The 7cm risers were used in later models (again, AFAIK) and are often sold as used parts. No other modifications are needed for any of the three sizes.
You might try ebay or register here and put up a search request in our
marketplace.
Keep in mind though that even with the higher risers, the handlebars will still be swept backwards at the same angle as before, and if it's the angle that causes your discomfort, you should probably consider installing a superbike handlebar instead. This is not very hard to do by yourself, but it needs a little more time, parts and money... Plus there are some minor modifications needed as for example the brake hose might not be long enough...