I get them from AliExpress, because it's super cheap there. Some sellers only offer very standard prescriptions (same diopters on both eyes, standard pupillary distance, no astigmatism correction). Others also offer to create glasses with custom prescriptions, just like you'd get the glasses from the optician.

Zenni Optical also offers custom prescriptions, but is more expensive (likely still a lot cheaper than your local optician).

But you can totally also get glasses with a lower prescription from your regular optician, but as you will relatively frequently need new glasses, that can get expensive (at least here in Germany it would, and I heard in the USA glasses cost up to $1000).