You can make your own: To make buttermilk, use 1 cup milk and 1 tablespoon lemon juice or white vinegar. Allow it to sit for a few minutes. If you do purchase buttermilk and have extra, freeze it in 1 cup containers or in Ziplock bags. You can also purchase powdered buttermilk in grocery stores.
Straight-line depreciation allows an equal portion of the laptop's cost to be claimed in each year over the total depreciation period. If the cost of the computer was $1,000, for example, then $200 a year can be included in the company's total depreciation amount each year for five years.
Assuming you are working an a single-site domain (eg, GPO changes are replicated in less than a minute), you can configure your computers' GPO background refresh (Group Policy Refresh Interval for Computers) to something like 1 minute so that they will receive the updated policies when this threshold is reached.