Where are the grounded 737 Max planes being kept?
They're at whatever airport they were at (or next landed at) when they were grounded. Airports are big enough places that there's room plenty of room to park planes that aren't currently in use, and it's completely normal to have such planes parked at an airport. Just look at a few in Google Maps satellite view.

At least in Europe (and presumably elsewhere), the directive banning flights does allow one ferry flight (of up to three legs) to get the plane to a location where any corrective action can be taken. But note that this does not allow moving the plane just to get it to a more convenient storage location.

Great Circle Mapper's featured map for 14th March showed the actual location of the 737 MAXes owned by US airlines, as of the previous evening.  Those aircraft are spread across around 30 airports in the US and Caribbean.



Thanks to Antzi for pointing out the ferry flight exception.