I need help finding and killing Planterra

Finding another bulb can take time. One spawns every in-game day and it's possible you've exhausted your bulbs (Plantera bulbs will not spawn until all Mechanical Bosses are defeated). It helps to clear out long level stretches for plants to grow on (as George Willcox noteS) and keep them cleared out to encourage bulb spawns. At least the time you spend molding and shaping your jungle will yield more bulb spawns!

As for Plantera, many usual strategies apply.

1. The Arena You say you've cleared out an arena, that's good. I assume you've got layers of platforms you can jump between? If you haven't done that, do. Have you stocked the arena with passive healing sources? Camp fires and heart lamps really help keep your regeneration up and should be standard features in any arena. Make sure you add some if you have not.

2. The Potions are you using any potions? You should be using Ironskin, Regeneration, Swiftness at minimum. Siftness does not stack with boots, but it allows you to free up an accessory slot if need be in many boss fights. Said slot can go to more defense, damage, or utility as need dictates. You should have the Well Fed buff, pumpkin pie is an easy and excellent source. It provides a boost to many statistics as well as regen. Thorns won't hurt - you, that is - and every little bit of damage helps. An Endurance Potion is an excellent idea but they're more difficult to make - requires fishing and patience. Potions that increase your damage with your chosen form of offense can help too (be it melee, ranged, magic, or summons) though I frequently skip them. You should also be using Greater Healing potions for heals, are you?

3. The Gear

Obsidian shield (or Cobalt but might as well make it Obsidian) is mandatory as usual. There are times when Plantera is going to put you in bad position and the only alternative is to move through him. You'll take damage but the knockback prevention will allow you to do it.

Wings are another must IMO so you're covered there. Just use the best you have access to.

Jumping/falling accessories - (ballons, horseshoes, bottles) are not necessary. Your wings should allow you to get to whatever heights you need to and they prevent falling damage. Ditch the horseshoe balloon for something else.

Boots - you can go with swiftness and use something else here as well, if you want.

Charm of Myths - a great accessory for any fight. More frequent potions and even more health regen.

As for what else to use. . . an emblem of your damage type is a good idea. So are upgrades like the Avenger Emblem (remember you can stack the Avenger Emblem with any of the base emblems). If you want to melee, the Fire Gauntlet is very good. There's nothing wrong with using multiple types of weapons on him but you can arguably benefit more by gearing for ranged or melee only. Though some people like to make ichor bullets/arrows to apply the Ichor Debuff to bosses. It reduces defenses.

One other accessory to consider.

The Cross Necklace - makes you invincible for longer after being hit. Combined with the Obsidian Shield this really helps out in boss fights. Star Veil is fine too but the falling stars are just as likely to get snagged on the top of the arena as not. It's the Cross Necklace that makes the upgrade shine in any case.

And lastly, how is your money? You might consider reforging for defense or +%damage onto your accessories, and aiming for something really good on your weapons (the best enchant for a weapon depends on the weapon type. It's usually godly for melee though). 87 defense is a lot, so offensive modes on your accessories might be the way to go. Reforging can get very expensive but can be a huge help.

4. Plantera Specific Tips There aren't many, alas. I find doing a summoner works really well, honestly. But I have typically employed the Optic Staff for this and you might not have a black lens to make one. When I do summoner, I make sure to get at least 5 summons (and I use low defense summoner armor). The spiky balls that Plantera produces will fall through wooden planks, as you have doubtless noticed at this point. They hurt the most of all of his attacks and it's best to avoid them if possible.