As you are performing internal audits on behalf of laboratories for their ISO 17025:2017 management system purposes, not on behalf of a certification body, ISO 17025 Lead auditor certification it is not mandatory. It is important, however, if your customers’ require it; or you see it as a means to further develop your skills and career. As per ISO 17025:2017 clause 6.6, Externally provided products and services; the requirement is that as an external provider, you should be competent (able apply your knowledge and skills) to perform the contracted tasks and provide a service that meets the laboratory’s need. The laboratory must communicate to you the reason they are contracting you and the qualification and competence required. This should include if they require you to have lead auditor certification or not.
If you choose to obtain ISO 17025:2017 Lead Auditor certification; select a suitable course offered by an approved Training Partners of an international certification body such as Exemplar Global (formally RABQSA) or IRCA (The International Register of Certificated Auditors).
As ISO 9001 is incorporated into the ISO 17025:2017 Management requirements and ISO 17025 Lead Auditor certification courses may not be as readily available as ISO 9001, you may consider starting by enrolling in the free ISO 9001 Lead Implementer Course. It is available at https://advisera.com/training/iso-9001-lead-implementer-course/.