it's possible a guitar repair shop has a gear hanging around... if you go there talking about dohicky's and thingofmabobbers however the chances are your totally going to get hosed
"Guitar Tuning Keys
Guitar supply shops frequently carry "Key sets"- the knob itself is called the Key. Key sets usually come with 6 Keys, 6 gears and 6 screws [LOL]. And I dunno it there's a different Euro term for them. I looks from the pic that you have a Nylon string guitar. If this is so, specify at the shop. The one you want (if you have to buy a full setup) is imaged repeatedly in the above link. at the very least, you'll have two backup keys/gears/screws if you replace the whole bracket.
If you wanna try Thurnez' advice, just tell them you had a gear come unscrewed from your pegboard, and then ask for just the gear/screw.
Around here (California) a lot of shops are nice enough to answer questions if you bring in the item. prolly because then, even if you don't know
what you're talking about, they can see
what you're talking about.
I have no idea how shops are in europe, generally.