Hello everyone, i'm trying to buy some altcoins (some of them are not really famous for the moment) but i can't find a marketplace where to invest, i tried to sign up on bittrex, but impossible to do it for the moment (too many people on it), do you have good marketplace ? thanks 
Cryptopia is the one I recommend. I also recommend you not to use:
-Poloniex
-HitBTC
-Shady exchanges
Bittrex might be worth it if you're able to verify your ID, though I've seen some accusations of them freezing accounts for longer periods of time.
Cryptopia is a small exchange though, so it might not have the best prices possible available and it doesn't have all cryptos, though you should be able to invest in the majority of them.
Openledger is another exchange I recommend trying out. It doesn't require ID-verification either and has a reasonable volume for some cryptos. Some cheap buys & high sells are available there too, as it's apparently a relatively unknown exchange.
There's also a lot of information about it on these forums and you might be able to find an ANN thread about the crypto your want to invest in.
EtherDelta is available for tokens you might be interested to invest in.
Have fun!