There is a list of companies and individuals that offer MediaWiki development for hire, and commercial support, at the page professional development and consulting. You can also find MediaWiki hosts and "wiki farms" at hosting services.
Other such lists, and MediaWiki-related job postings, can be found at the following offsite pages:
- mwusers.com: MediaWiki Market
- Semantic MediaWiki: Professional support
- oDesk: MediaWiki contractors
- WikiMatrix.org:MediaWiki consultants
- MediaWiki professionals listed in LinkedIn.
Please note that these sites are independent, and not affiliated with or endorsed by the Wikimedia Foundation.
Within reason, it is also acceptable to post MediaWiki-related job openings on the mediawiki-l mailing list.