Polymer Bundle is an uncommon resource in Warframe and is used to build weapons, Warframe components, Archwings, and gear items. It is a very commonly used resource in the game, so you will need to farm a lot of it.
The Assur node on Uranus is a great place to farm for these resources. Assur has two advantages for farming. First, it is a Dark Sector mission with a buffed resource drop rate, and secondly, it is an endless Survival mission.
These are some of the easiest missions for long farming sessions. You can find a spot on the map with one entrance, then stay there and all the enemies will come to you. Running a team with a Nekros will allow you to use his Desecrate ability to re-roll the loot tables of dead enemies, increasing the number of resources you will get. You can also run Warframes like Hydroid with Pilfering Swarm, or Khora with Pilfering Stangledome to increase your loot drops. It is a good idea to have one person bring a stealth Frame. They can leave the room you use as a base and get the life support capsules without dragging away any enemies, or just getting destroyed by the number of enemies that will be just outside the door to your safe room.
For these missions, it is also a good idea to farm for as long as possible. Every five-minute rotation will give you an improved percentage chance buff, and some of these affect resource drops.
Polymer Bundle drops are also affected by resource boosters that you can buy from the market place. It can be a great idea to pick up a three-day booster to increase the drop rate of resources, and a three-day booster to double the number of resources that you collect. Dedicate a weekend to doing any farming that you need to do, and you can then enjoy a few months without needing to do much farming at all. Finally, if you have a Smeeta Kavat, its Charm ability will increase the number of Polymer Bundles that you collect.