You acquire them from BfA World Quest Pet Battles. PvP, I’m afraid, is entirely useless for rewards.
On any day, we get 4 pet battle World Quests in BfA - 2 in the morning at reset, 2 more 12 hours later. Each lasts 24 hours. So if you log on at the same time every day, you will always be able to get 4.
On average, it seems to me that 2 of the 4 reward Polished Pet Charms, somewhere between 13 and 20 PPCs per quest. So on average, you should get about 25-40 a day per character. We do get dry spells, where hardly any Charms are awarded for days, and then sometimes, all 4 quests reward Charms. RNG at work.
Xu-Fu’s provides strategies for all the BfA WQs, if you need them:
https://wow-petguide.com/index.php?m=BattleforAzeroth
However, most of those strategies were formed with the idea of levelling a carry pet, and in 8.1 the devs removed almost all pet XP, so not worth bothering with the specific strats if you just want the Charms.
I did all the BfA World Quests with a team of
Ikky
Zandalari Anklerender P/B
Iron Starlette H/H
and completed all except Rogue Azerite with just those three pets. BfA has extremely easy world quests, AFAICS, and other than Rogue Azerite and some retries on Captured Evil and Not So Bad Down Here, these three steamrolled everything.
I used a P/B Anklerender just because. I’m pretty sure that a P/P or S/S would be at least as effective.
I used a H/H Starlette because it survives AoE in the back line better and because of the extra health when it resurrects after Failsafe. I think a P/P would probably be at least as good, though, because Wind-Up -> Wind-Up on its own without Supercharge is so much more effective.
More about that here:
https://www.warcraftpets.com/community/forum/viewtopic.php?f=3&t=17902&p=141624#p141624
The maps on Wowhead don’t have the right descriptions at the moment, but they do correctly show when Polished Pet Charms are available as rewards.
I see right now that Desert Survivors in Vol’dun is up rewarding Charms, but it will go away at reset.
There are also occasionally follower missions that reward PPCs. I wouldn’t want to be depending on them, though; they’re not very frequent.
Do please ask again if this doesn’t clear it up for you!