I have a problem with one of the Thieves Guild quests. This concerns Tonilia. I once received an armour from her, but i selled that armour, don't know to who anymore. Now i have to give her one piece armor back in order to receive a new armour, but since i don't have the previsous anymore, i can't finish this quest.

Look at the bugs list at the bottom of the link http://www.uesp.net/wiki/Skyrim:Tonilia

I think you are doomed unless you have an earlier save, maybe if you are on PC you can do something with the console.
why you sold it btw? there's a thousand way to make money

Unless you have an earlier save you're stuffed I;m afraid. Every few weeks I see somebody coming onto the forum asking for help with this. Why you'd WANT to sell it is beyond me, but you DO need it.

Never sell special armor like that. Even if you do not wear it, you can always put it on the manequins at home. I actually dawn the Guild Masters Armor...I rather like the look...

Some unique stuff stays at vendors, if you are not on PC then maybe just scour every shop for it.

Why on earth would you sell it in the first place? The only remedy is to reload a previous save to before you sold it if you're on console. If you're on PC, then you're lucky.