Start by keeping a list of the DVDs you need to buy. Having that on you every time you go out for shopping will make it easier to prioritize without the need to recall whether you already have a particular DVD stacked up somewhere in your library.
Online shopping can be the best way to buy cheap DVDs and build your collection. Ebay is the outright choice for most due to its popularity and more importantly, reliability. You are bound to find most of your desired DVDs at a reasonable price. However, before buying, make sure you keep external factors into account, such as shipping and handling costs. You don’t want to pay any extra money for movies which you have already watched, even though it is among your favourites. Moreover, you may find other websites or stores which are selling these DVDs on the cheap.
Apart from ebay, there are numerous websites you can shop from. These include, buy.com, Amazon, Title Trader, Deep Discount, eCampus etc. Comparing the relative prices will give you a fair idea about the pricing strategy used by all.
There might be a couple of used DVD stores in your area, which may just be willing to help you out. Although they are close to redundancy, plainly due to the increase in online transactions, you can certainly get your favourite DVDs at a relatively low price. Moreover, places like Walmart and Kmart often use discount techniques to sell off their old stock. Frequently visiting may just help you get the right deal.
It is often the case that your friends will possess collections of their own but are not movie freaks like you. Swapping, exchanging or even paying just a little may help you entice them into giving away their DVDs, further allowing you to build your own collection.