It should be

"They get what they want"

 and

"When they need help, they get it."

They has a singular case, but that shouldn't change the rest of the sentence. The key here is to recognize that you don't know who or what they is. They could be congress, or Steve from accounting. "They get what they wants" is just poor English. No amount of teaching it that way will ever fix change that, and if you leave the "s" on the word "wants" when using "they" in the singular case, you're going to sound like a moron.