how do you get TF2 KEys? in game or where?

you buy them from the tf2 store in steam, for 2,49$

You can get them from the steam market, buy them from the steam store or buy them from other people (cheapest but also more risky) from trade sites.

but how can other sell em cheaper thatn 2,50 bucks when you get them for 2,49$ ???

The only way they're added into the economy is by people buying them on the TF2 store at full price. This is generally done by
A. people who don't know better
B. people looking for a cheap way to premium while still retaining the bulk of their 'investment'
C. people using fraudulent purchase means

They then sell at a loss on the market place for Steam Wallet or to traders for cash. This is then circulated into the economy. Keys are a bit of a universal currency on Steam, people are willing to trade keys for not only other TF2 items but even things like Steam games, trading cards, etc. TF2Outpost has a pretty substantial key trade marketplace, where you can buy or sell nearly anything on Steam for keys.

SourceOP forums, they cost around $1.8 or less
just be careful

You can get them on the the Mann.Co Store for 2.50$ on the Market (it's cheaper there), or on tf2 warehouse.