They are.

Indeed, "Aus einem Geschäft?" is accepted.

Because is a prepositional phrase aus is a dative preposition forces the sentence in dative ein=einemthe Noun Laden is masculine

I hope that helps

I am having the same trouble

"ein", "einen", "einem"...etc are the word for indefinite articles, e.g. "a/an".

Nominative case: masc/fem/neuter = ein/eine/ein
Masculine - "Das ist ein Hund", This is a dog
Feminine - "Das ist eine Katze", This is a cat
Neuter - "Das ist ein Buch", This is a book

Accusative case: masc/fem/neuter = einen/eine/ein
Masculine - "Ich habe einen Hund", I have a dog
Feminine - "Ich habe eine Katze", I have a cat
Neuter - "Ich habe ein Buch", I have a book

Dative case: masc/fem/neuter = einem/einer/einem
Masculine - "Ich komme aus einem Baum", I come from a tree
Feminine - "Ich komme aus einer Schule", I come from a school
Neuter - "Ich komme aus einem Haus", I come from a house

Resource: https://en.wikipedia.org/wiki/German_articles

Hope this helps :)

www.lsa.umich.edu/german/hmr/Grammatik/Basic_Chart.html