There are two things which might work, depending on what the other fit requirements are.
If buying a larger overall jock gets a big enough pouch, but the waistband and or leg straps are then too long, start out with this oversized jock and shorten the waistband and straps. This can be easily done by cutting and sewing up the back of the waistband to shorten it, then shortening and reattaching the leg straps so that they are in the right position and the right length. This is far less work and expense than making a jock from scratch. If you are not handy with a needle, most dry cleaners, uniform stores, and similar establishments will do simple alterations like this very inexpensively. Just fit the jock and use pins to hold everything in place, then bring it in and ask them to sew it.
A second possibility assumes that you are looking for a soft-pouch jock, not one with a cup. Some jocks designed for cups have oversize pouches to accomodate the cup. Constructions vary greatly here, so look at various styles. You may find such a jock that has a big enough pouch when worn without the cup.
An additional possibility is to have something custom made. If you go to a surgical supply store, I am sure that they will be able to have a "surgical appliance" manufacturer make a jock-like supporter to custom measurements. However, this is likely to be expensive, and may not conform to your idea of what a jock should look like. From time to time, some LPSG posters have siad they have had jocks custom made by athletic goods manufacturers. Perhaps one can post exactly how they went about getting this done, and who they contacted, as it seems unlikely that most mass-market manufacturers today would be able to accept custom orders for small quantites. Most of their stuff is now produced in huge lots offshore, and I've never had luck getting anything (shirts, etc) special ordered.