Can anyone help me with were to find information about building a private indoor shooting range. We have a customer that would like to build a range in a warehouse/office building in OKC.

Thanks

tunnel range, large concrete pipe/culvert 25 meters long with enclosed range house at firing end and enclosed butts at target end with lights and extractor fan

For internal modifications or remodeling, check with the OKC Code Enforcement if it is within the OKC limits for what is needed.
But your biggest problem will be the EPA nazis. They've been known to shut down gun shops over bluing tanks.
When I was at the OKARNG 5 years ago, all of the indoor armory ranges were shut down because of EPA issues.
I avoided anything that had to do with the EPA so I can't really tell you what they would require.

I have ordered the information that the NRA has available. That should give me the basic construction info. What we are having trouble finding out is who inspects or certifies that the range is built correctly and what documentation do you need to allow you to discharge a weapon in the city limits. The OKC building inspectors do not have any requirements related to a shooting range other than what would be required for a room that size (8' x 125').
I thought there might be some LE guys that could steer us in the right direction.

If you want to avoid the very expensive HEPA filter system required, you can run a green ammo only range. This will affect your shooter market to some degree.

Edited to add, if it's truely a private range, don't let EPA on the premiss. NRA has plans for indoor ranges.