I'll trade you a Gavantula, but it'll take awhile for me to catch one with the Swarm ability. I accidentally traded my Gavantula with Swarm away , but if you just need a regular Gavantula, I'll be ready to trade.
before, there was an IV calculator, and if you input in your stats and nature and any known IV's. it tells you what they could be. another way is fighting keckleon and using hidden power check what type it changes.