When there is a government shutdown, I know DOD personnel don't get paid, but how about spending on beans, bullets, fuel, drugs, etc.?

If a government shutdown occurs, the government may not make any authorization, obligation, or expenditure of monies they don't have (without a new appropriation bill, most government agencies are only funded until midnight on 30 SEP). However, if the money for the contract has been funded already, then there is minimal effect on that contract (fixed-price contracts, multi-year contracts, etc.).

To your specific question, no - suppliers cannot take IOUs. Even if they were willing to do so, the contracting authority is forbidden to do so. The government won't be able to award new contracts, extend existing contracts, modify contracts or do anything else with money that has not already been appropriated. Additionally if you had an already approved contract that is funded periodically throughout the year, there may not be funds available for them to continue.

HOWEVER, if the contract covers something that is deemed and "essential activity necessary to protect life and property" then OMB does provide guidance on continuing or obligating new funds to those items.

If there is a shut down then only things that have been paid for will come in, 0 new orders, 0 refill orders!