Buy them on eBay or Amazon. I highly recommend swapping out the PS4 thumbsticks for XB1 thumbsticks - they're better quality.
I bought these, and found a video on Youtube explaining how to tear the controller apart.
It probably took me about 20 minutes. Just be careful not to lose the spring in the triggers, otherwise those are screwed.
I have these and theyve been flawless for 6 months. I dont like the material they use for the stock ps4 thumsticks as after a while it gets sticky or slippery. I havent had any of these issues with these.
Can I use these?
Well I cant promise theyre original, but here is a link.
http://www.amazon.com/2-Pack-Controller-Analog-Sticks-Dualshock-OneSoul/dp/B00IZSTNKW I used these. Not metal, but they've survived my destiny addiction.