get is grammatically correct, but be is very much more common in passive voice sentences. This NGram graph shows that usage of get is negligible with offered.
get is normally used to indicate a change of state, whereas be is used to indicate being in a state. For example, get ready is what you do to change from not ready to ready, whereas be ready suggests that you have been ready for a while.
get is far more likely to be used in passive voice sentences in circumstances where somebody is seen as a victim of circumstance (got hit by a car, got dumped). See this NGram graph and note that i) was is still a lot more common than got ii) got is more common in US English than British English.
get may be more common in informal British English, but that's hard to prove.