Zeraora is not being officially distributed anymore, but if you already have a code, you have until January 24, 2019 to redeem it ingame. If you don't already have one, your best bet would be to go to an online trading forum and trading for one that somebody else hasn't redeemed yet, or you could trade for a Zeraora that's already been redeemed (although I don't recommend doing the second one).
It's going to be very difficult to get one for free, but if you don't have anything to trade for, then it might be possible. About a day or two before the deadline to redeem Zeraora comes, some people may give away spare codes for free on trading forums (I've seen this happen multiple times on the PokemonTrades subreddit), although this is a very risky strategy since there's no guarantee that people will have spare codes, and if they do, they generally get taken very fast. If you really want Zeraora, I wouldn't wait for giveaways except as a last resort.