I could be mistaken however I do not believe Moto still makes shells for the Moto G1 or G3. I don’t see anything about the G1 shells on their website and all of the G4 colors are listed as out of stock-- I would be rather surprised if they end up getting any more of those in at any point in the future. For the G4 family, I don’t believe Moto ever sold the shells by themselves; only through Moto Maker at the time of purchase.
I’d recommend checking out Amazon or another retailer; I managed to find a fairly wide range of colorful options for the different generations of Moto G in just a minute or so of searching!
Edited for typo
If you’re looking for them for the Moto G4, there are tons of options on eBay for about $8 each.
If you’re looking for the Motorola Grip Shell for the Moto G 1st generation phone, you can still find some at Amazon: