- Only now you can even get them on top of wrinkles.
- Only infrequently does it happen.
As one of our members has said, inversion happens when a sentence starts with "only" and never otherwise. So why does no inversion happen in the first sentence?
When I first saw only now you can get them I though it was ungrammatical, because, as you say, only normally triggers inversion.
However, it is grammatical on a different reading. In everyday speech, only can also function as a conjunction, with a meaning similar to but. There will be a different stress:
Only now can you get them
with even stress on only and now, means "You can get them now, but you could not get them before".
Only[,] now you can get them
with only unstressed and probably rather quick, and possibly in a separate breath group (represented by a comma in writing), means "Contrary to what has just been suggested, you can get them now".
Inversion happens when a sentence starts with "only" and never otherwise. Eg:
Only if you come with me, will I go there.
I will go there only if you come with me.
This is true of many other negative adverbs like "never", "under no circumstance", "rarely" etc.