Here we have to have an Eng. draw it up and get approved by city. You might try your city hall are the eng. that drew it up are the builder, depends on how it was done, did you build your own house are buy it, it will be hard to get a copy, what do you need it for.
If you cant get any info then you will have to dig around until you find it. You might try http://www.google.com and ask them for septic leaching field layout and kind of go by that are try the septic tank co. and see if they can give you idea how field is layout.