Im Supermarkt finde ich keine Mandeln mehr.

Because this sentence describes location, not movement.

• in + dative = being in a place
• in + accusative = going into a place

So:

• im Supermarkt = in the supermarket
• in den Supermarkt = into the supermarket

Here, the speaker is already in that location, so German uses the dative.

Also, im is a contraction of in dem:

• in dem Supermarkt → im Supermarkt

German often puts a time, place, or other adverbial phrase at the beginning of a sentence for emphasis or topic.

So instead of starting with the subject:

• Ich finde im Supermarkt keine Mandeln mehr.

German can also say:

• Im Supermarkt finde ich keine Mandeln mehr.

Both are correct. Starting with Im Supermarkt puts special focus on where this is happening.

Because German main clauses usually follow the verb-second rule.

That means the finite verb must come in the second position. If something other than the subject comes first, the verb still stays second, and the subject moves after it.

So:

• Ich
  • finde
    • ...
• Im Supermarkt
  • finde
    • ich
      • ...

Here, Im Supermarkt takes the first position, so finde must come next.

Mandeln is a plural noun, and German often uses kein/keine/keinen... to mean no / not any with nouns.

So:

• keine Mandeln = no almonds / not any almonds

Because Mandeln is plural and is the direct object here, the form is keine.

Compare:

• Ich habe keine Zeit. = I have no time.
• Ich sehe keine Kinder. = I see no children.

Because after keine, you do not use another article.

• keine Mandeln is correct
• keine die Mandeln is wrong

In this sentence, keine already does the job of a determiner. It works a bit like a negative version of ein-words.

Compare:

• Ich kaufe Mandeln. = I buy almonds.
• Ich kaufe keine Mandeln. = I do not buy any almonds.

Mandeln is in the accusative case because it is the direct object of finden.

The verb finden usually takes an accusative object:

• Ich finde den Schlüssel.
• Ich finde die Bücher.
• Ich finde keine Mandeln.

With plural nouns, die Mandeln is nominative plural or accusative plural, and here it is accusative plural.

Here, mehr means any more or any longer.

So keine Mandeln mehr means:

• no almonds any more
• no more almonds
• not any almonds any longer

It suggests a change from an earlier situation. Maybe the speaker could find almonds before, but now cannot.

This is an important difference:

• keine Mandeln mehr = no almonds any more
• Mandeln nicht mehr finden = to no longer find almonds

In this sentence, keine negates the noun:

• keine Mandeln = no almonds

mehr adds the idea of any more / any longer.

By contrast, nicht mehr usually negates the verb or whole idea:

• Ich finde Mandeln nicht mehr. = I can’t find almonds any more / I no longer find almonds

In many contexts these can be very similar, but keine Mandeln mehr is especially natural when the focus is on the absence of the almonds.

Here finden means to find, in the sense of to be able to locate.

So the sentence means the speaker cannot locate almonds in the supermarket any more.

But finden can also mean other things in different contexts, such as:

• Ich finde das gut. = I think that is good.
• Wie findest du den Film? = What do you think of the film?

So learners should remember that finden can mean both find and think/find something to be depending on the sentence.

The singular is die Mandel.

So:

• singular: die Mandel = almond
• plural: die Mandeln = almonds

In this sentence, the plural is used because the meaning is general: the speaker cannot find any almonds.

Yes. That is also correct.

Compare:

• Im Supermarkt finde ich keine Mandeln mehr.
• Ich finde im Supermarkt keine Mandeln mehr.

The difference is mainly focus:

• Im Supermarkt ... emphasizes the location
• Ich ... is more neutral

German is flexible about what comes first, but the verb-second rule still applies in main clauses.

Yes, it is natural and correct.

It sounds like something someone might say when talking about availability in a store. Depending on context, a native speaker might also say:

• Im Supermarkt gibt es keine Mandeln mehr. = There are no almonds in the supermarket any more.
• Ich kann im Supermarkt keine Mandeln mehr finden. = I can’t find almonds in the supermarket any more.

But your sentence is completely normal, especially if the speaker is talking about their own experience of looking for them.