A carpet cleaner should, at the bare minimum, have at least two companies that they can refer clients their clients to when they have such a requirement.... That being;
1) A Plumber
2) An Exterminator
Both of these companies should be those with which a reciprocal relationship exists, so that they will refer their clients who need the services of a carpet restoration professional, to you.
I'm sure there are other specialties as well for which you should have a reciprocal referral relationship, if you do not offer such a service, e.g. a carpet repair specialist, a color repair technician, etc (unless sub-contracting is an option).