MyOrgan does not use Soundfonts. I am not aware of any free large organ sample sets available other than those sample sets listed on the MyOrgan homepage. Won't the Stiehr-Mockers organ listed there do?
If you want cheap three-manual sample sets you will need to go for a different system: jOrgan. This is free, but requires a bit more work to set up on your computer. Also, the sets available at the moment are synthesizer-developed rather than true pipe samples. But they can still sound good if you can use plenty of reverberation. jOrgan differs from MyOrgan in that it does use Soundfonts. I believe there is a large neo-baroque pipe organ sample set listed on the MyOrgan homepage which is also available as Soundfonts suitable for jOrgan use. It is not free.
The Miditzer mentioned in the previous post is also free, and is easy to set up, but it is a TPO (Theatre Pipe Organ). Some of its sounds are very good.
There are many large organ sample sets available for Hauptwerk, but they and the system itself are certainly not free, and may not fit well with people on a "tight budget".
MyOrgan, jOrgan, Miditzer and Hauptwerk are all capable of producing excellent sounds.
John Reimer