a) spare part (the catelog is called 'spare part'; it's a coinage)
b) spare parts catalog (The catelog sells spare parts)
a) spare part solutions (again, it's a name, a coinage)
b) spare parts solutions (the solutions refer to spare parts; this isn't a name or a coinage.)
"spare part solutions" is a very cool name for a catelog, and "solutions" is a cool name for a department.