Sometimes at my workplace we come across some really obscure bugs or issues with technology that we use, e.g. HAProxy, or database sharding / replication, and nobody on the team has enough experience to diagnose what's going on without spending a few days getting really deep into the technology.
In situations like these we'd like to a find true expert with the technology, someone who has dedicated most of their time to it, and we'd be happy to pay $300+ an hour for their expertise. Most of the time we don't even need code written, we just some guidance on what could be broken and how to fix it. Usually we need help immediately too.
Unfortunately I can't find any sites that have these kind of freelancers. I remember hearing about a site like this a few years ago on a podcast, but I can't remember the name. Normal freelance websites take far too long to post the job, filter all the bad candidates and maybe find one person that knows what they're doing.
Example: "HAProxy"
Go to http://stackoverflow.com/tags/haproxy/info and on the right hand side, you will a section called top answerers. Each of these people have spend a good amount of time answering questions on the topic you are looking for and then have been up-voted by a community of peers as having reasonable answers, that work, and that help people solve their issues.
To "HAProxy" in particular, we find that "Willy Tarreau" is the top answerer. Doing a quick google search ("Willy Tarreau" HAProxy), we find on Willy's about page "My name is Willy Tarreau...Today, my work mostly consists in designing and auditing robust and scalable hosting infrastructures. This has led me to develop the HAProxy load balancer, which is used in several large infrastructures with high level of demand concerning availability, performance and security."
There is a good probability that if you wrote Willy an email saying, I'm happy to pay $300+ for an hour of your time that you'd get an answer, if not a potential introduction to someone who can help.