You don't need a purchasing department to buy smarter. Most of it comes down to asking more than one supplier, asking the same way every time, and writing down what you paid.
Send the exact same list, with the same quantities, to each supplier. If the list is different, the totals don't compare. Ask for unit prices per line, not just a total.
Let suppliers offer equivalent brands, but compare specs (gauge, rating, listing, warranty) before choosing on price.
Save your last invoice for common items. That's your yardstick for whether a quote is good.
Many suppliers and manufacturers run contractor rewards. For example, the National Apartment Association describes a Home Depot program with a potential 2% annual rebate on qualifying purchases for members above $25,000 a year (source: naahq.org). Terms vary, so read the program rules.
Competitive quotes are normal. Tell your main house when someone beat them. They often match, and you keep the relationship.