There are several options available ,,you can either have copies made from other existing keys or you can have the locks that the missing keys operated rekeyed.
Basically the lock is taken apart and the internal key code is rebuilt to a new code with matching new keys.
This is the safest option especially if your concerned that who ever finds the lost keys might know which property they belong to and may illegally enter your home.
The average lock cyclinder has about 75000 usable key codes , so you can see that there is a lot of scope to change the keys
Call steve at Arrow Locksmiths today for more advice if you are in this situation.