Here's a pretty easy way.
Step 1: Magnetic Personality
Go to New Mauville and wrangle up a Magnemite. They're in the mid-20s. I suggest grabbing one with Sturdy as this will help later. Be sure to give it a swanky nickname like Artoo, Cyclops, Polaris, etc.
Step 2: Movin' on Up
You're going to want to raise your Magnemite up to Lv 41, when it learns Lock-On. It is imperative that you do not evolve it into a Magneton because then it will take you until Lv. 49, and who has time to wait that long? I suggest using Blissey Bases to level up your Magnefriend nice and quick. Having it hold a Lucky Egg is even better, you can find them on wild Pelipper.
Step 3: Fite Children
Once your Magnemite has learned Lock-On, put him and Smeargle at the front of your party and Fly to Mauville city. Go west to Route 117 and battle the Twins you see there. They'll have a Hariyama and a Linoone (hence why Sturdy is good to have)
Step 4: Etch-a-Sketch
If Smeargle is faster than Magnemite, use some random move and then have Magnemite use Lock-On. Next turn, use Sketch on Magnemite.
If Magnemite is faster than Smeargle, just use Lock-On and have Smeargle Sketch Magnemite.
Step 5: Profit
If all went well, Smeargle now knows Sketch!
Alternative Method
Catch a Remoraid and use a Heart Scale, it learns Lock-On at Lv. 6, considerably earlier than Magnemite. However you should probably still level it up so that it doesn't get KOd by the Twins.
