Here is where you can some Montag CDs, merch and more:

The Blue House
You can buy things directly from Montag (not the actual retail CDs but exclusive EPs, merch and stuff).

Carpark Records' shop
Go shopping on Montag's label's site...

Rallye Records' shop
If you live in Japan, you might want to try Rallye Records instead.

Otherwise you can buy the records from:

Here
Here

But we all know that the best place is your local independent records store, of course, of course..