If you prefer a physical copy or are looking for a deal on a used version, retailers like Noble Knight Games often have copies for around . You can also find various listings for the original 2005 box sets on eBay ranging from $10 to over $40 depending on the condition. Why Avoid "Free" Downloads?
Your best bet for a legal, high-quality download that actually works on modern versions of Windows is to grab it from a reputable digital storefront. Official Digital Stores
A re-launched version is also available on the Steam Store for a similar price. This release was specifically updated for modern hardware. Physical Options (Collectors)