Try here:

http://www.hometech.com/brains/timers.html

http://www.ics-timers.com/op4142.html

Or "Google" for "12 volt time delay relay".
That's how I found them.

www.mcmaster.com and search for timing relays. You're looking at $45
minimum, but these are industrial relays and if you order by 4pm weekdays it
will ship out same day

 I usually use Mouser for small orders

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=5
89431&e_categoryid=440&e_pcodeid=52801

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=5
54918&e_categoryid=440&e_pcodeid=52834

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=5
54980&e_categoryid=440&e_pcodeid=52834

As some examples (ya I know you aren't likely to be switching 300A @ 12V)

What is the application?  All you need is a capacitor across the coil
terminals to provide delay.  I've done this for different reasons.  It just
takes a bit of experimentation.