Hi
Depending how much you like your watch to look nice and genuine, you have several choices:
1- The most obvious and most expensive choice is a visit to IWC boutique or AD and they will get the strap for you perhaps at the price you paid for the watch itself!
2- The second option is to only buy the Quick Release buckle from them and use a generic rubber strap which would still look nice if the right strap is chosen.
3- The next choice is to look on auction sites and see if someone is selling a second hand one. This is a cheaper option but if you are someone like me too fussy and worried about hygiene then perhaps not a good choice.
4- Finally, if you are only wearing the watch for sake of time keeping without taking it off your wrist to hand it over to friends for show off (I am sure you are not that kind of guy ;)) then you can get a generic quick release buckle and rubber strap which in naked eyes cannot be distinguished with real one (unless of course you hand it over). This option can cost you as little as 10 bucks.
5- Finally the free option, yes, you read it right, "FREE" option would be to keep the metal bracelet which in fact I think it is very nice with your watch.
Whichever option you choose, best of luck and happy hours with it.
Best wishes