Is it possible to get an eye exam done in a foreign country and come back to the US with prescriptions for glasses and contacts and have that filled by ideally one of the online retailers?
Knowledge from the comments on the question, no personal experience here.
You can use a foreign prescription for glasses on several of the online USA glasses sites, but not on all.
And some will have restrictions on which prescriptions are acceptable, like an expiry date.
But warning, if your glasses or lenses need to be 'non standard' because your eyes have 'non standard' problems, you best go to a place where they check the prescription to your eyes and measure and adjust the glasses to your head or check the shape of your eyes for your lenses.
A slight offset of the glass in the glasses will make them hard to use, or even worse.