Maintaining certification

I would like to know if:
a) a consultant is necessary to achieve/maintain ISO standards

Answer
It is not mandatory to have a consultant to achieve or maintain ISO certifications. The only requirement is being compliant with ISO standards and internal requirements. Of course, in your particular situation perhaps a consultant can be useful for maintaining the certifications, because she or he already has an overall idea of how your Integrated Management System works and how things are interrelated. Another possibility can be to use the consultant to train you and other people about the Integrated Management System.

b) it is mandatory to audit the company's supply chain and if so how?

Answer
Nothing in the standards make that mandatory, unless your own internal rules define that as mandatory. Remember, internal rules can be changed by organizations if they are not effective or if context changes.

The following material will provide you more information about maintaining certification and supplier evaluation:
How to maintain your ISO 9001-based QMS after certification - https://advisera.com/9001academy/31/how-to-maintain-your-iso-9001-based-qms-after-certification/
How to evaluate supplier performance according to ISO 9001:2015 - https://advisera.com/9001academy/blog/2015/10/27/how-to-evaluate-supplier-performance-according-to-iso-90012015/
- [Free webinar on demand] - How to integrate ISO 9001:2015 and ISO 14001:2015 - https://advisera.com/9001academy/webinar/how-to-integrate-iso-90012015-and-iso-140012015-free-webinar-on-demand/
- [free course] ISO 9001:2015 Foundations Course – https://advisera.com/training/iso-9001-foundations-course/ [there is also a free course about ISO 14001:2015]
- [free course] ISO 9001:2015 Internal Auditor Course - https://advisera.com/training/iso-9001-internal-auditor-course/
- book - ISO Internal Audit: A Plain English Guide - https://advisera.com/books/iso-internal-audit-plain-english-guide/