Definitely no cotton or anything dyed. A lot of hardware store strings(even sisal or other fibers that should be safe on their own)can be treated, same with jewelry string.
A good example for those in the USA/Canada(not sure if they ship elsewhere):
https://www.rawganique.com/hemp-ropes-p/hr6a-1kg.htm
I would definitely recommend that one, being organic and European-grown(and made) and 100% untreated(no harmful pesticide residues, no unsafe coatings or chemical treatments). It is more expensive initially as far as the price itself goes and you'd need to pay shipping, but it is a worthy investment that should last quite a while as it is ~154 feet of rope.
More feet but more of a narrow option if you'd prefer that:
https://www.rawganique.com/organic-h.../tw12b200-.htm