Willfescon,
It sounds like you're doing a lot of service work. You may want to try something that I did a few years ago for similar reasons. I wasn't putting together large enough orders to quote them all individually and was buying from whichever supply house I happened to be closest to at the time so wasn't really doing much volume with any single supplier.
I identified my high usage items and their total expected usage over time and asked my suppliers for a fixed per unit and also a quantity (per carton etc.) price quote based on that info. The idea being that they would honor that price for a period of time i.e., three months to a year and in return I would give them something close to the stated volume of business.
I received quotes from three different suppliers for this and one of them actually gave me some pretty decent pricing over what I had been paying over the counter. I did have to watch their invoices though as they would sometimes invoice at a higher than negotiated price. They will tell you how often they will want to re-quote based on their cost change history and will of course exclude things like wire.
This also made estimating jobs a little easier.
