Hello Ingo,
I have been looking at the Prague exhibition myself, but more in the sense of the different exhibitions. The 1908 one has a great abundance of postcards, which makes it possible to get a good view on the different exhibits. For the older exhibitions the amount of different cards is much lower.
I also found two original advertisements from that period. They are nice additions.
Milan Cernik has great offers, and you can also find nice items on Ebay and Delcampe of course. Another auktion is Profil in Prague:
Aukce Profil - Úvod (filatelie.cz)
Best regards
Edwin