THE COMPLETE ASSHOLE'S GUIDE: GETTING FREE SKETCHES AND MAYBE MORE
Step 1:
Make an account on Deviantart and/or the Furaffinity forums.
It helps a lot if you let your account sit for a month or longer before using it.
Step 2:
Go to the job offers forum and state you're hiring. Be vague with the details. Very soon you will be swimming in dozens of artists desperate to be hired.
Step 3:
Scan the artist's commission details. If they say they take payment up front, do no business with them.
If their details say nothing, or if they state that the sketch comes before payment, send them the details. Do this to as many artists as you can.
Step 4:
You'll soon have a number of sketches. Some artists will even send you a full illustration in advance. If they send a watermarked image, they can usually be removed with a bit of photoshop knowledge (especially on B&W images or flat colours). If the image is a low resolution, throw it through waifu2x.
Congratulations! You now have exactly what you asked for, hopefully.
Step 5:
Cancel your commissions. It's best to do so tactfully and in a way that prevents the blame from being shifted onto you. State you cannot accept the sketch the way it is due to anatomical errors, perspective issues, not being close enough to what you were envisioning, etc. You can even criticize their ability which will typically evoke a defensive reaction, at which point you can just outright state you don't want to work with someone so temperamental.
Alternatively, say you don't like the sketch, perhaps imply it's outside of their ability, and then ask if you can have them do a different picture instead. Effectively, you'll be getting two for the price of none.
IMPORTANT NOTE:
This is effectively scamming art from someone. This is horrible, and you should not do this. Treat sketch scamming as you would pirating an album, movie, or game. If you like it, pay for it when you can afford it.
Just kidding. Fuck 'em. Make another account and incubate it for a bit. Then, in between step 2 and 3, message all the artists who gave you free sketches in the past. Chances are that they'll still be easy marks the second time around.