How you go about this depends upon the particular requirements if your project. Some options could include:-
Whatever you have hanging around. Empty ice cream tub etc.
Ready made enclosure. Hammond , Boss etc avail from digikey, mouser, rs, farnell or others.
Sick of beige laser cut acrylic.
Custom laser cut plastic.
Low quality 3d print. Makerbot etc.
High quality 3d print. Shapeways etc.
Home made milled from solid. If you have access to a mill.
Low volume CNC milled from solid. Protolabs firstcut or others.
Low volume injection mould. Protolabs protomould or others.
There are also lots of services for front panels etc. Many people use PCB for this.
I’ve also seen lots of professional products use Peli cases or similar with custom panels cut to fit. You can even buy mounting packs to go inside.