Simple cruise untill you see something you like the look of.
Novatech.com is a good source, as indeed others too many to list.
Type PC Tower Cases into Google- you will see.
One thought however Modern cases are not a patch on older ones in construction and material used.
My own 64 bit machine- has gone through many incarnations over the years- but the case is original and about 8 years old and built like a battleship.
So a look around the secondhand market might be worth a look- even if you have to gut the innards.
Try also the magazine MicroMart - on sale weekly from the usual sources