There are some small simple ones that have Band-aids and alcohol wipes and then there are very advanced Trauma kits. You can buy a simple kit and then add things to it to make a better first aid kit.
Here is a minimum recommended list of stuff that should be in a kit.
Assorted adhesive bandages
Assorted gauze pads
Antibacterial spray or ointment
Scissors (Trauma Shears)
Other things that can be very useful:
- Sanitary Napkins
- gloves - (medical)
- CPR Mask
- small flashlight
An over the counter First aid kit will work fine and so will putting one together on your own. I have a "Trauma Kit" that I have all the supplies in a Gallon Ziplock bag.