I know this is an older post but I did run across this when I was looking for something else.
Go to https://guildsofwow.com/looking-for-guild
Select: Region, Realm and Faction.
Sort after: By Achievement Rewards.
Press the Achievement Rewards button on guilds and go down the list an see if they have Challenge Warlords: Gold - Guild Edition. If you find some that has 25/25 they have everything. But some with 23/25 og 24/25 could also have it.
Now just contact the guild ingame or make a request to join. Some guilds might be inactive.
NB. You need 300g on the character you need to buy it with.
Edit: A tip: If you can’t find a guild on your server, and decide to create a level 1 on another server that has the achievement. You can buy a pet and cage it and sell it on the new server. Might take a while…
Hope it helps and Good Luck!! = )