If it’s a decent screwdriver it should already be hardened. The Wiha and Wera models are very good, but you do pay for that.
I realise this is region specific, but the screwdrivers that Lidl get in from time to time are actually really good and super cheap. Miles better than the own brand ones I’ve got from Screwfix.
Seconding @anon29631895’ Wiha recommendation. Another option is professional jewelers’ supplies, such as http://www.riogrande.com/Search/screwdriver — Which are also not cheap, but I suspect that in addition to being more fragile and generally crappy, cheap tools are more easily lost because the owner doesn’t treasure them. Like most people, I used to regularly leave umbrellas god-knows-where, couldn’t hold on to one for more than a month, until I finally got a nice umbrella. I’ve had it for years now.
ETA: Fuck those stupid plastic cases. Get or make a tiny tool roll if the drivers don’t come with one. You can make the whole thing out of duct tape and thread.
I have using those screwdrivers. I can never get a good grip on the handle and they hurt my hand. I picked up a set of the $4 ones with the green tops at HF and they work fine for my occasional needs.