"In the ********" or "At the ********"

I'm not a profesional translator, I'm a designer, but because I have to read all the texts after making them all pretty I sometimes find mistakes translators miss. I'm kinda stuck on a preposition today, help!

The original text reads:
El paciente recibe un mes de suplido de medicamentos especializados en una farmacia de su predilección.

The translation I got skipped the preposition "en", so now I'm trying to figure out if it should read "...at their ******** of choice" or "...in their ******** of choice".

I vote for at, but I'm not sure. Is there a rule that says which one should go where?

EDIT:
Mmmmm, one more question guys. How do you feel about the word "get". I feel it's out of place in corporate communications, but it could just be me. I hate seeing sentences like "...which allows you to get your medicine by mail."

I agree with you, that it should say "at."

I also agree with you that the word "get" is too informal, although the trend in the English language is that it's becoming more informal as times goes on, even in corporate communication. Something like "obtain" would be a better choice.

Agree about the at and in the other sentence you could also use the verb receive. Just another option.;)