The answer to where is less important than the answer to from whom, because the one thing you cannot do with a solar viewer is check it yourself. The supplier is the evidence, and everything else follows from that.
Start with the checked list
The American Astronomical Society maintains a list of suppliers whose solar viewers it has verified. Buying from that list is the only positive evidence available to a consumer, because the ISO marking on the packet is printed by whoever made the product, which is precisely the thing in question. Counterfeit markings are widespread before every eclipse.
What buying locally gets you
In the US, science museums, planetariums and astronomy retailers stock verified viewers around an event, and buying in person lets you inspect the filter before you leave. In the UK stock spikes around a visible event and vanishes between them, which is when marketplace listings from unidentifiable sellers appear, so buying well ahead is the practical advice.
Buying online without being caught out
Buy from the manufacturer or from a retailer that names the manufacturer. Avoid listings where the maker cannot be identified, regardless of the review count, because reviews measure delivery rather than infrared transmission. If a listing shows an ISO number and no maker, that is the whole assessment.
Inspect before every use
Hold the viewer up to a bright indoor light. You should see nothing except the light source itself. Any scratch, pinhole, crease or separation at the frame means the pair is discarded, not used carefully. This takes five seconds and it is the only check a user can actually perform.
Questions people ask about eclipse glasses
Where can I buy eclipse glasses?
From a supplier on the American Astronomical Society's checked list, or from a named and traceable manufacturer. Science museums and astronomy retailers stock verified viewers.
Are eclipse glasses sold in shops safe?
Only if the maker can be identified and checked. The marking on the packet is not evidence, because counterfeits carry it too.
How do I check eclipse glasses are safe?
Buy from a checked supplier, then hold them to a bright indoor lamp: you should see nothing but the lamp, and any damage means discard them.