18mm motor retainers are now available from Rocketarium.com. These are aluminum, anodized, and made in the USA.
They are designed for use with 18 AT RMS reloadables, as well as single use Estes/Quest/AT model rocket motors. Price is only $11.95 for a full set.
In addition the 18mm and 24mm motor retainers, a 18/24 adapter is now available.
This lets rocketeers fly 18mm RMS reloads, and 18mm single use motors, with Rocketarium.com 24mm motor retainers. Dealer discounts are available.