Stinging nettles use a complex effect to produce the sting you feel, there is an irritation caused by the hairs, (known as trichome) and the effect of several injected chemicals. The trichome are actually hollow and carry mainly histamine (the same compound that is produced in an immune response) and other compounds which causes a burning, itching and swelling.
If the reason you are allergic to nettles is due to the histamine (it commonly is) then you really have few options here as pretty much anything plant (or insect) based that produces a similar effect will cause the same kind of allergic response.
Options that will not incur this nasty response would be to use a purely mechanical sting such as flicking with rubber bands or hits with a thin cane. You could also explore using very high voltage shock devices like a Violet Wand. These can generate a shock that stings in the same way the static shock you get touching a door knob after walking on new carpet can do.
