The East Kingdom Youth Combat Rules are the ultimate arbiter of what armor is required, although it can be sometimes be difficult to interpret for newcomers without additional context, so you may want to ask a marshal for assistance.
See the Armor Bag Checklist page on the East Kingdom Youth Combat web site.
Below are some notes I’ve shared with parents in the past, but remember that there are multiple ways of meeting the requirements, and this is not an endorsement of the specific products mentioned below.
You can outfit a Div I fighter with new sports gear for about $100, or you can spend less if you find used items or hand-me-downs.
For helmets, the standard recommendation is a youth hockey helmet, which you can find used (“Play It Again Sports” or equivalent) or at a sporting good store or online.
The “Bauer Prodigy” hockey helmets are decent and can be found for around $55, although other nearly-identical models sell for closer to $100.
If you can bring your kid to a store for test fitting that makes it easier to settle size question, but if you’re going to a store without your kid, try to measure around their head above the eyebrows, and write down that dimension in both inches and centimeters, as you may see either printed on a helmet.
There’s an online guide to choosing an appropriate helmet.
Gloves are required, but almost anything will do: cloth gloves, or thin wool winter gloves, or thin leather. Just make sure they’re not slippery, so your kid can still grasp their sword.
Groin protection is easy to pick out at any store that sells sporting gear. Alternately, you can construct a padded skirt or apron with sufficient protection.
Gorgets are harder to find, but you can find padded cloth neck protectors in the youth hockey section, or online. I picked up some youth hockey neck guards for $12 at our local sports shop, which are basically a long band of black fabric with a bit of padding in them and some velcro to fasten the ends together around the neck.
Elbow and knee pads are optional for Division I, but easy to find if you want them — cycling or skating pads are a good choice.