Just go to poloniex.com, click on EXCHANGE at the top. On the right hand side, there is the exchange market. You don't even have to be logged in.
At the top there are four orange tabs:
These make up valid coins that can be the first part of the pair.
Once you click on one of the tabs, there's a list of coins. Every coin listed is a coin that can be the second part of the pair.
The reason you don't see LTC_ETH is because LTC isn't a valid funding coin. IE, you can't buy anything with LTC as the funding currency (it's not an orange tab at the top). Likewise, you don't see ETH_LTC because when you go to the ETH tab, LTC isn't one of the coins listed on that tab.
