There is an order form on the last page of "The Pneumatic Manual for FIRST 2002 Competition". "Custom" is used in the context of being able to select a particular size of cylinder to meet your needs not in the context of being able to design your own cylinder.
You can order up to 4 using the order form and the pneumatics documentation (including the order form) can be downloaded from the FIRST site.
The only available bores are 3/4", 1-1/2", and 2". Strokes range from 1/2" to 12" (there are some selections that are not available).
All 4 are Free.
WARNING: 2" bore cylinders are VERY powerfull but VERY HEAVY.