We use Cintas to stock our first aid cabinets. In my opinion, it is overpriced and you pay for convenience. If the safety cabinets are 5S with the items that are needed, this could easily be ordered in house just like the office supply cabinet.

We maintain our first aid supplies internally.  Each first aid cabinet or backpack is inspected monthly.  We have a monthly "Building Inspection form." Each supply is listed.  If it needs replenishing it is noted on the form.   Expiration dates for eye wash and AED pads are checked monthly, and noted on the form.  Replacements are available the month of the expiration. 

We also replenish during the month if needed but at least each kit is inspected monthly.