The file to use is rc.firewall.local. Note that any script can go here and be run after iptables loads. It does not just have to iptables rules. In principle put all your IP's in a file then write a mini bash script. I'm not at home so cannot look at any of my scripts or do any testing, but you'll want something like:
for IP_TO_BLOCK in $(cat your_block_list_file) ; do
iptables -I INPUT -s $IP_TO_BLOCK -j DROP
My script writing has a lot of trial and error in it so you may find my for loop is not remotely correct but I hope it gives you the idea.
Scriptlet checked and quotes removed from for line.