The springs are small and easy to not find. They are not always lost. Sometimes they stick to the magnetized P-90. They are not easy to install either. I really can't imagine them coming without springs.
Also for some reason, aftermarket pup covers don't usually fit well. Sometimes it takes a LOT of grinding to get one to fit. The easiest solution is to use genuine Gibson covers.
In a quick search on ebay, I couldn't find any springs specific to this installation. There's a lot of Fender springs that look like it but it's hard to tell their size from a picture.
There is a picture of of what you need on the following link. The springs are in the top left corner of the picture.
USA Gibson Les Paul 60s P 90 Soapbar Pickups Pots Knobs Jack Assembly Cream | eBay