If you want Windows 7 you need to buy it. You reasonably qualify for retail Builder's OEM version since the laptop is not completely built yet without an OS. It gets a considerable price break at around $100 US.
If that is more than you can afford now then your options are to use free Linux which has many choices but they are not exactly consumer friendly.
Or you can install and give a test run to Windows 8 which is in beta phase right now. It's not recommended to run beta versions on a "production machine" which is one you rely upon for use however it is as stable as Windows 7 and includes many improvements. The only difference is it starts up to a touch-screen type interface that function as clickable buttons on a non-touchscreen device. It takes some getting used to, maybe customizing, but it is free and as a beta-tester will get you in on deals like half-price pre-sale or free Signature editions for hosting a preview party later when it is nearing release.
If you're interested visit our sister site Windows 8 Forums
to get started with excellent support, or ask back here.