Don't hold your breathe - it seems that Walther/Umarex parts are very very hard to come by.
Chambers have limited stock of parts and their website says bolts are out of stock. Dealers seem to struggle to get support from Armex (who are the official importers), but there is a number to ring on the Armex website under the Service Tab - good luck!! I have asked about parts on here and as a result tried a few European dealers too - still no luck.
They will have to get their act together before I would ever consider buying another Walther/Umarex.