I think Steam is trying to tell me something. :-p

(Though in all honesty, I’m not sure what.)

Where’d you get the booster, if it’s not rude to ask?

Booster packs are randomly awarded to you when you use Steam. It is rare, but it happens.

You can also buy it in the Marketplace, or trade for it with someone who has one.

It’s not rude at all! :-)
You can buy them on the Market, but I got mine as a random drop.

How can I get a booster pack?
Once you have received all of your card drops, you become eligible for a booster pack, which is a set of 3 trading cards that may include both basic and foil cards. Booster packs are granted randomly to eligible users as more badges are crafted by members of the community. Make sure you log in to Steam each week to maintain eligibility.

Once eligible, your Steam Level increases your rate of receiving a booster pack drop: [ul][li]Level 10: +20% increase in your drop rate [/li][li]Level 20: +40% increase in your drop rate [/li][li]Level 30: +60% increase in your drop rate [/li][li]Level 40: +80% increase in your drop rate [/li][li]Level 50: +100% increase in your drop rate (i.e. the rate has doubled) [/li][li]Etc.[/quote]- Steam Community :: Steam Trading Cards

So basically, you play the game for a couple of hours or however long it takes to get all the Steam Trading Cards that you’re eligible for and then gradually they’ll come in. (You don’t have to be playing the game to get that game’s booster pack.)

If you’re not sure whether you’ve got all the card drops for a game, you can check on the Badges screen. (For me that’s either Steam Community :: Steam Badges or Steam Community :: Steam Badges . Replace part of the URL with your own ID or Profile number; or you can view it in the Steam client by going to the top where it’s got STORE, LIBRARY, COMMUNITY, {NAME}, and select Badges from the dropdown list beneath your name.)

Steam is now trying to sell moving pictures to us! Not even devils would try something so insidious!