You can't buy a "realistic imitation firearm" (aka "RIF")without having a suitable defence listed under the Violent Crimes Reduction Act.
Such defences are museums, theatre, films or reenactment. You need to supply evidence of your legit activities to prove it.
The only other exemption is with airsoft, if you are a member of a skirmishing site. Without this, you can only buy the brightly coloured ones (because brightly coloured is not "realistic") It is an offence to "manufacture" a RIF, including painting a non-RIF replica to look like a real one....
Offences carry the standard threat of 5 years for a firearms offence, so I wouldn't push your luck.
However, you can purchase a deactivated weapon, its not a replica and is not covered by the VCRA.
They can be expensive, depending on what you buy though.