Most Airsoft guns use the metric sized allen key system.
So M1, M1.3, M1.5, M2, M3 and so on.
Most basic allen key sets dont generally cater towards M1, M1.3 and M1.5 as they are very small but are also the most common sizes in airsoft.
You can get these sizes from DIY shops or eBay
If however you have a Polarstar Fusion Engine, then you will need a imperial set.