Go to the "yellow pages" and look under "safes". There is an excellent chance that you will be able to find an old cabinet safe that will keep 12 rifles safe from thieves and fire, but plan to install your own gunracks after you remove the shelves and files.
They generally run less than two rifles ($1,000) and are an excellent buy.
Expect to pay an extra couple of hundred dollars to have professional safe movers deliver the safe, and roll it into your garage.
Also, if it's an older safe, have the lock mechanism overhauled. That's another $50 or so, but well worth it.
Varmitgun is right: You will outgrow that safe!
There is so much stuff to put in there: coin collection, silverware, cameras, passport, you name it. When you have a safe, there is no point in leaving valuables out.