Valley Shop located in Zypher Paradise and you need to visit Upper Valley. You can only buy couple of Upgrades.

Upgrades

Upgrades that you can buy from Marshall's Shop

Name	Description	Effect	Source	Price
Pelletspleen Mod	This extra pelletspleen adds more firepower to Gus' primary shots, giving your attacks a bigger spread.	Increases Gus' damage	Marshall's	1500

Video

For directions, Please watch the video below.