I've already got two Win7 Pro disks in my case, and all I need is another license. The computer shop is about an hour away, and I'm feeling lazy.
Is there any way to purchase a license key without media?
Yes, although you can also install Windows 7 without the product key and it will function perfectly fine for 30 days.
Sure. Do a search using your favorite engine and you'll likely find many sources that offer only the key. But be wary of non genuine or pirated keys. If the price is too low to be reasonable, it's most likely a pirated key. The last time i looked, Microsoft was offering licenses without media.