Typically, 7-speed cog spacing and 8-speed Campagnolo cog spacing are the same (5.0mm) ...
- Shimano's 8-speed spacing is supposedly 4.8mm, but the 8-speed wheels are generally interchangeable if the high-low stops are adjusted for the difference in the offset from the centerline.
If the as-is spacing doesn't work then you can certainly get spacers from WHEELS MANUFACTURING.
The extra spacer(s) can be from any salvaged cassette(s) ... if you feel you need incremental spacers, someone once measured the aluminum in a soda can as 0.2mm.
BTW. Initially, before removing a cog, just swap shifters and see how well the 7-speed indexed shifting works.