it might be a good idea considering to order things like panniers and racks from the internet, as far as i know you can get them cheaper online as in stores.
i liked "velophil" a bicycle store in alt mohabit, berlin, but it was pretty expensive though.
then i know "zweirad stadler" which claims to be the biggest bicycle store in germany (after i have been there i'm pretty convinced its actually true, its quite huge). although they lack of atmosphere they have very reasonable prices. they had as far as i remember a huge collection of bicycle supplies of all kind. only panniers they had little (only ortlieb and vaude i think), but for a journey i would only buy ortliebs anyway (from the internet, you can find them much cheaper there). but thats just my opinion.