Hi, you can handle most things like this in a standard will...just as you would assign your personal posessions to a spouse or other...and I definately recommend it! It could be a friend, family or even a specific rescue group...whatever you want.
I have a friend whos mother recently passed away and did not leave details for her pets care after death, the result was a bitter fight between the family members, one wanted to just take the animals to the local humane society (shame
) and my friend wanted to take them herself. I have even known people to have clauses in their insurance policies and detailed out the requirements of care to the inherited family member.
We are currently working on our wills and they will include care for our babies should we "go" together. This is something that is often overlooked by petowners, and it's great that you are preparing for the possibility. Best of luck!