A tip on finding manuals: google with some of these
"installation manual" product-name
manual product-name pdf
product-name manual homebuilt
product-name pdf
where product-name is say GNS430. The last one in particular digs out PDFs from deep inside certain US avionics distributor websites; ones they never knew were online
Using double quotes is an important trick.
The homebuilt phrase is good because a lot of US homebuilders have written up their project on a website and have put all the PDFs online on there.
Garmin have recently been hitting all websites carrying their installation manuals, threatening legal action. I've got a big collection myself (PDFs but not online) and I am happy to trade so if anybody has any, get in touch
Every time I come across any manual, I download it and save it.