Mods, personally, I don't use them, but the mods thread should tell you which are good and which to be avoided, if you want to use them.
The pink diamond is available spawned on the map, sorry, I can't remember exactly where, but, I think it is in the guide on this forum, there are maps of most of the towns which will show you the rough locations. If you tell us which map, some here will probably be able to point you to the right places.
Other places and ways of getting them:
The consignment store. (If you don't mind being a little crafty, put a couple of extra tills in the store to increase your chances.) Another way to increase your chances is to buy up everything in the store forcing the shop to restock.
The board breaker - you need to master martial arts and break 150 oak boards for this to work. You may have to break just a few, or an awful lot of space rocks before you get one. You will get a wide variety of different rocks this way, some quite valuable.
The miner - make one on the invention bench, or buy one. Again, you might get one on your first dig, but, more likely you will need to do many digs. You will get a wide variety of different items this way, some quite valuable.
Testingcheatsenabled true - Buydebug -place an epic gem spawner on your lot, or anywhere you prefer and wait for one to spawn. Needless to say, this is outright cheating. You will get a wide variety of different rocks this way, some quite valuable.
I believe that there are other ways, but I don't have all the EP's.