You cannot battle someone with Treecko, Grovyle, or Sceptile but there are a few exceptions. You can battle a secret base trainer (it must be the base's owner) with a Treecko, but the chances are pretty low. Your rival can have a Treecko if you chose Mudkip as your starter Pokemon, but then this question would be a bout Torchic. The only way you could get him is trade. You could try nicknaming a Pokemon "Treecko?" and trade with random passersby, if they accept the trade show them the Pokemon that is nicknamed "Treecko?" and they might get the message that you need a Treecko. You could also get on the DB's chatroom, Im sure there are tons of people that could get a Treecko. (Hints to myself) Your last option is the GTS this will be a multiple step part.
1. Breed your Torchic/Combusken/Blaziken
2. Hatch the egg you bred.
3. Get on the PSS, select the folder icon at the top and find the icon that says GTS, then select it.
4. Once in the GTS, deposit your Torchic into the GTS.
5. When you select the Pokemon you are going to ask for, scroll all the way down and click "What Pokemon?". Next, type in "Treecko" in the box, make sure that it is spelled right!. (It doesnt matter if it is all caps or all lowercase.)
6. You can also select what level you want the Treecko, but it would probably be faster if you select "Any level."
7. Wait a couple hours, then check the GTS, if you have gotten a trade it should show you the trading sequence, if it doesn't you haven't gotten a trade. Just check every few hours or so and it should happen eventually.
Hope I helped!