I usually stick to Amazon.com, Amazon.co.uk, Flipkart and Landmark-on-the-net for purchases.
Blu-ray.com is an excellent source for hunting deals. I maintain an Excel sheet of movies that I would like to buy and then I put a target price on Blu-ray.com (you need to be registered with the site). For instance, I want to buy Les Misrables. The Amazon.com price is $23.98 whereas UK price is 15.00. Both are too high in my opinion. I have placed a target price of $12/8 on blu-ray.com. If and when the target price is reached, I get an email.
Then again, on Amazon, I try to look for deals such as "Blu-Rays under $10" or "3 Blu-rays for 17". Sometimes, you find wonderful deals this way. But shipping cost is a killer. For example, on Amazon.com, assuming an order of 6 BDs, the average shipping cost per BD is $4.82. If the BD is $9.99 and shipping cost is $4.82, it is nearly 50%. Ouch!!