I hear this a lot, usually from people who have difficulty finding interesting postcards where they live. I get most of my postcards from bookstores and small boutique-y places, of which there are plenty in Seattle. I don't go out specifically looking for postcards (because I have so many at this point I could open my own shop), but if I see one that appeals to me, I buy it.
When traveling, it's easy to find postcards - but the challenge is to find good postcards. There is no shortage of location-specific postcards to be found at every major attraction, and if you're totally out of time, the airport or train station will do. But trust me, there are better postcards out there representing your travels. I fall into this trap time and time again, buying an okay postcard set at a tourist-centric gift shop, only to find MUCH better postcards later in my trip at a small independent shop.
If you live somewhere and can't find any fun postcards, here are some places to get your fix:
Hold the Mustard Postcards - a small outfit with great postcards. I bought some of their postacrds in the '80s and am thrilled they are still going. Support them!
Stella Mars - interdisciplinary artist from the Pacific Northwest who also designs her own line of postcards
Postcardly - make your own postcard from your photo! I have not used this service yet, but received a postcard from someone who did.
I will update this list periodically ...