how do you find these???

The super rod is found in the battle area in a house just north of where you get off the boat that takes you there initially. Many people seem to have trouble finding it.
You wont be able to catch a garchomp per-se, unless you cheat. But yes you are right, to get a garchomp you must first catch a gible down in wayward cave, the entrance of which is hidden underneath cycling road. Ill leave you to find it. Be wary though, gibles will be like level 13-15 down there so plan accordingly.

You can find the Super Rod in a Fisherman's house on the Battle Area of Battle Island. In order to be able to get to Battle island however, you have to beat the Elite Four, and obtain the National Pokedex by seeing all 150 Pokemon on the sinnoh Pokedex. After you have accomplished those two things fly to Snow point City, and ride the ferry to battle Island. Head north upon landing and you should see a bunch of houses, in one of them will be the fisherman that gives you the Super rod. You can find a Gible in the secret area of Wayward cave, which is hidden under a portion of the Cycling Road. You can look up a video on youtube.com for it's exact location because I did the same thing. After you catch one just train it to level 24 and it should evolve. Level him up to level 48, and he should evolve once more into his last form, Garchomp.

Hope this helps.

Yes you are correct. You are going to have to catch a Gible to evolve it all the way to a Garchomp.
To catch a Gible, their should be a bike trail under the town that you first meet Cynthia in. Under this bike trail should be a hidden cave to which you can not see the entrance. In this cave you should find a Gible. Hope this helps.