Thetford makes a toilet riser that you can use with any RV toilet.:
Dometic and Thetford RV toilet models:
RV Toilets - Dometic
Tweetys.com usually has very good internet prices on RV products and here's their toilets to give you an idea on cost:
Do you know the actual height you want? Maybe you can find one that is close to the height you want without using a riser? I know some RV toilets are taller than others and they make a "high profile" model. If you presently have a plastic toilet, now might be the time to go with a porcelain toilet. Or maybe you can just install a riser. I don't see a Dometic riser.
There is a flange adapter available so different toilet makes can be used:
By comparison, a standard residential toilet height is around 14.5" - 15.5" floor to rim, so an RV toilet at 17.5" is a lot taller. Just 2" makes a big difference.
Unless you get this done at a shop, make sure you replace the gasket between the flange and toilet even if it looks okay.