As others mentions here, I also use the sketches from the sales brochures, plus pictures I find on the web as basis for creating my own drawings. In some cases I also get hold of the parts catalogues for further details if needed.
There is a publishing company in Germany that sells many different drawings, made by private persons, of RC models for scratch builders, but when it comes to excavators it is very limited.
http://shop.vth.de/bauplane-frasteil...fahrzeuge.html
I have to mention that many sketches in the newer CAT sales brochures are out of scale, doesn't make sense if one start scaling and taking dimensions. I had to give up the brochures and need to take a trip to a dealer with the tape measure
But new Volvo, Hitachi, Komatsu brochures that I've been studying are quite 'dead on' when it comes to the sketches.