When I meet with my Sysco rep, I am armed with Sam's and GFS pricing. While she is looking up to "see" if they carry it, I will tell her, "I can get it at GFS for $4.99 a can (or whatever), so meet it or beat it." Information is the best tool you can have. The sales reps have the power to special price for you. And yes, some places are better than others. Example: I was doing a breakfast for 100 college guys and needed the 10 oz bottles of orange juice. Sysco told me "You'll probably get a better price with GFS because we don't do a lot of business with schools and hospitals."
I go thru the same thing with my Sysco rep. I remind her that I get lots of ideas from walking thru GFS so for her to limit me to what SHE thinks I need is not good business. I show her the monthly flyers I get from GFS and told her it would be great to get these from Sysco. She said, "I didn't know you wanted them."
Having been in sales and marketing for years, and having had sales people report to me, I told her it was her job, as my sales rep, to let me know what was available. If Sysco didn't want to "take care of me", then obviously GFS did.
You just have to remind them that you are informed and there are other options. That's why it's not a good idea to limit yourself to just one supplier.
Funny story: During Katrina, Sysco was out of 50 lb bags of sugar. I usually picked up at least 2-4 bags a month from them. When she told me they couldn't get sugar, I said, "Well, you should back your truck up to Sam's Club because they have plenty!"