Well if you want to buy some LTC for your BTC you should indeed like it was told before create a thread and place it into #marketplace subforum
https://bitcointalk.org/index.php?board=161.0

Title it "[WTB] ammount of ltc - will pay in BTC"  or something in a kind of.

And if you want to use any online services it is hard to believe you don't know any yet

Sometimes best one depends on where do you live.

I am using bter.com  if I want to change some coins, but you can prefer other one.

There is a lot of them. You should read:
https://bitcointalk.org/index.php?board=84.0
and
https://bitcointalk.org/index.php?board=85.0

there you can find a lot of online exchanges that will help you to exchange BTC into LTC.

Regards