For Your Inspection: Source Code For Photoshop 1.0
The Photoshop code can not be compiled/linked without also having a copy of Apple's MacApp framework. Since the code is written in Object Pascal, you would probably need version 2.0 or earlier of the MacApp framework in order to compile the code using MPW.
How Do I Get Open Source Programs Written For Me?
In my experience, there isn't a single place to find Mac OS developers.
Posting a job opening or project on rentacoder.com or dice.com is very often like looking for a needle in a haystack.
Two groups that specialize in Mac OS development jobs are:
As to programming rates, it varies with experience and which part of the world you are dealing with. If you are dealing with programmers in the USA, you will have to pay higher rates for programmers working on the East or West coast because the cost of living is higher. Don't expect experienced programmers to work cheap either! An experienced programmer with 5-10 years of experience will start at $50/hour, with a typical rate of $75-$100 depending on project length and difficulty.
Remember, a good experienced programmer will do the job right the first time. An inexperienced programmer will sometimes take several tries to complete that task and the resulting program will be fragile and difficult to maintain.
A quick check for determining programming experience is to get a development estimate for your project specification. Give the programmer a complete project specification (including screen mock-ups) and have them give you a project development estimate. An inexperienced programmer will typically under-estimate the time and difficulty of the project.
If you are developing "general-purpose, scientific programs developed and released as open source", you should check the BioCocoa site to see if your project can leverage work already done there. If you can do the work in Java, the BioJava project is a good place to look for BioTech related libraries.
Another good place to find more information about doing scientific research using Mac OS X is the Mac Research web site.