Where can I find training wheels that would fit into these slots:
All the training wheels I've seen online are ones where you have to attach by removing the hex nut on the center of the back wheel.
This is a Huffy 16in bike.
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The training wheels that come with the bike are designed for easy assembly on a new bike. They are not sturdy and the only way I know to find a replacement is to get one from another bike, maybe someone who's kid didn't need them anymore.
Some local bike shops will carry 16" training wheels, usually sturdier than stock, and they should install them for you for about the price of a flat tire fix. That would save you needing a wrench set along with the hassle of installation.
The last option is to buy them from somewhere and install them yourself. Maybe a neighbor will loan you some tools.
It's always good to check the bearing adjustments in the rear hub, front hub, crank, and headset as well as having the correct chain tension. These bikes are often adjusted incorrectly from the factory. It can make a big difference on ease of pedaling.