Early purchasers of Super Smash Bros. Ultimate were able to receive a free download code to play as Piranha Plant. If you registered your game on My Nintendo by January 31, you should have received a free download code via the email associated with your Nintendo Account. Keep in mind that this download code expires 11:59 p.m. PT on June 30, 2019. If you missed out on this limited-time offer, Piranha Plant is also available as paid DLC for $4.99 / £4.49.
Source: Nintendo Versus @NintendoVS