Unless you mean the welfare servants, there are sporadic "events" (usually X million downloads) where they give you a ticket which you can trade for any one of a select group of SR servants. This happens once annually I think. This year's has already passed, next years will happen in the latter half of 2019 and will have a better selection to choose from (all non-limited servants, including story-locked ones).
Here is a reference and list of available servants if you are interested:
https://fategrandorder.fandom.com/wiki/10M_Downloads_Campaign#SR%20Servant
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