Check online retailers, especially those who deal in discontinued merchandise lines. Amazon.com carries "Star Wars Trilogy (Widescreen Edition with Bonus Disc)," but notes that the manufacturer discontinued the product, released in 2004.
Look for the "Star Wars Trilogy" released in 2008, which contains a bonus disc with the original films included. If you cannot locate a limited edition, the re-release makes a good backup choice (also carried by Amazon).
Peruse online auction sites such as eBay to find either edition for sale by previous owners. Retailers who still have stock also list their items on eBay; it is possible to find a set new, in the box. Check back frequently, as merchandise varies over time.
Post an ad on an online classified site such as Craigslist, with specific information on the set you would like to obtain. This may match you with a collector of memorabilia in your state or country (even around the world) who has the set.
Investigate online fan forums; you may meet someone with an inside track to a collector or seller. Large scale conventions, celebrations and events are also rich resources, exposing visitors to collectors and sellers of series-related items, including DVDs.