typically niches are found or created, rather than listed on job searches. If you're already a contract software engineer, the most accessible 'niche' would be contract/interim CTO positions. Beyond that, specific expertise or contacts in specific industries would be the way forwards; at this point it's not a matter of applying for jobs, but creating opportunities.
[edit] forgot to mention contract (cloud) architects and project managers. They can make a lot, too.
[edit] forgot to mention contract (cloud) architects and project managers. They can make a lot, too.