Any large jobsite needs to have an emergency first aid attendant onsite, particularly if they are working in something like construction. You'll find that it is a building site requirement after you have a certain number of people working on the site and remote jobsites need specially trained attendants who can transport the injured.
There are services that provide emergency first aid workers for jobsites like this, where the job is expected to end within a specific amount of time. You can hire an attendant to work for you until the job is over and then they will move on to the next site.
If you are looking for first aid attendants, your first stop should be GetBiz, where you'll be able to find out which services provide attendants in your area and receive a quote from several of them. This can really be useful in helping you make your decision.