Reention keys (CORE-5, 500 Series, 700 Series, and LMT'd)
If you still have 1 key, you can likely have a copy made locally for less than $10.00. Your local hardware store or locksmith may have blanks and be able to make you a new key right away for a fraction of the cost. Ask if they have an HD-74 blank and to verify compatibility before purchasing.
Alternatively, if you lost both keys, you can order a replacement here: https://ride1up.com/product/replacement-key/