For me, I find W&G merchandise when I'm not actually looking for it - its like a 6th sense lol.
Best places in my opinion are car boots, theres always someone selling something W&G related. If your not too fond of car boots, Ebay would be a good place to go. There is always a few hundred items being sold, but this increases to the low thousands in the run up to Christmas. Ebay is good because if like me, you like to have lots of semi-collections within your main W&G collection - (mugs/badges etc), you can always find another thing to add to it, even when you thought you had the entire set. Happy hunting!