I am not aware of a wall-to-walllow VOC carpet(with pad) at the $3-4/sqft price point you have stipulated.
You could consider a carpet tile such as those offered by Interface's Flor.
- The tiles are easy to lay, made of recycled content, can be easily picked up for cleaning stains and have low/no VOC content.
- The tiles are approximately 19" square, so if you consider the Working Class or Fez lines you are looking at $3.60/sq. ft. before tax and shipping (and installation).
- Flor's Checkered Past, Finer Things, Mod Macrame, Mild Mannered, or Lacebark lines would run you about $4.40/sq. ft. before tax and shipping (and installation).
If you have a hard time deciding which line/color to choose, Flor will send you three 6" by 6" samples for $5 -- great insurance.
Their website is very easy to navigate (this is where you can go to order the carpet tiles/samples) and you can even design your floor on their website with "Floor Builder" -- a great tool for determining how many tiles you will need plus what tiles of different colors/patterns may look like together.
Interface is just one of many manufacturers who manufacture carpet tiles.
If you have your heart set on wall-to-wall carpeting, I would recommend you go to the Carpet & Rug Institute's website.
- You can look up their member manufacturers who have earned the Carpet & Rug Institute Green Label certification.
- Additionally, you can find pad manufacturers that have earned the Carpet & Rug Institute's Indoor Air Quality Green Label for carpet cushion products.