I barely know about this field. But generally you either.
1. Use automated audit service.
2. Hire an individual auditor.
3. Seek security company which offer security audit service.

You probably want to ask this question on either security/open source community or research how similar software/script got audited. For example, WALLET.FAIL which discover several weakness on hardware wallet is run by company called Keylabs. Take note it's just an example and i don't endorse them.