I buy my respirator and cartridges locally. I prefer the North brand as that's what I am used to from work and what I have been fit tested in.
You want the OV (organic vapor) cartridge for fumes not a HEPA cartridge that is only for dust.
I have several thousand hours wearing respirators In the Industrial cleaning industry.
They tell us change the ov cartridge every 12 hours.
The reason being is the harmful vapors you cannot smell. It could be used up and you not smell anything but you are breathing in fumes you can't smell.
The hepa cartridges are good until they start to get plugged. But they are only good for dust and not what you would want for soft plastics.
As was mentioned once you open the package they start to work and you cannot shut them off and even the good non harmful smells use up the cartridges.
Also it might be a good idea to have a fit test done. That will ensure the brand and size you choose fits your face properly.
If your not wearing the correct size or not wearing it properly you could be exposing your self to harmful substances and not even realize it.
I get my fit test from work every year.
Also keep your respirator clean and in a sealed ziploc type bag.