I'm Navy but we follow the FAR just like everyone else does. On smaller purchases under $2500 the first line of credit card holders puchase. Expenses above that require a contracting officer to execute.
Most items require sole source/brand name justification unless they're generic machines. If we spend more than a certain ammount we need to let the request sit for 30 days - $100,000 I think. Otherwise we get three bids and the contracting officer takes the lowest.
Small business, miniority, and disabled still have set aside business quotas dispite the fact that they were declared illegal.
I spend somewhere between $350,000 to $500,000 a year on computers, software and service. I visit the trade shows, shake hands and meet people. I get along with the less than pushy types who take my order, might suggest an addtional thing or two or pitch something better. I want quick answers and quick supply. Typically these sales guys have an engineer who answers the hard questions for them/with them.
If you've got catalogs make sure that they get sent out in the July/August time frame as the end of the fiscal year sees me dropping money that needs to be spent and planning next year's big budget items.
If you get to the trade shows invite the military guys to the after hours dinners/parties, get them some chochkee with your name on it and keep in touch with them - once a month if you haven't heard from me that month. Call with something new/exciting based on previous purchases - I buy lots of publishing, video editors and Macintoshes. When the next version of my editing program suite hits your shelves would be a nice time to call.
Good luck.