Best bet is a junkyard. The issue is going to be the length of the bolt.

If you broke the old one off most likely you had the 13mm nut and bolt on the arm tight or someone else got carried away in the past and you are the lucky one who had it break off. As many bent bolts as you see on 240's in Junk Yards, you know even the pro's get it wrong when it comes to getting belts tight.

Start out with all the 13mm nuts loosened, the most important one is the nut on the back side of the block that the 10mm bolt screws into. Start cranking on the 10mm bolt and you may have a long crank depending on the length of the belts. The 10mm should not be under any tension until the belt(s) get tight. As you crank in on the 10mm you are moving the Alt away from the engine along the slot in the arm. You do not have to pry, snug, or even worry about any other part of the alternator or belts, just crank on the 10mm bolt. When you get the belts tight and you are happy with the tension, then and only then do you start with the 13mm bolt and nut that hold the block in place and tighten it up. Then do the long bolt and 13mm nut and you are done. The belts should not loosen after you get the bolt tightened. You can take your time getting the other bolts tight if it is done right. If you see that one belt is tighter than the other, check and make sure your accessory bushings are not worn out or deformed. The one under the arm that you tighten against is the one that gets the worst of it. And since it is out of sight under that arm a lot of people miss it. You do not want one belt doing all the work, defeats the reason for having two.

This works on the AC and the Power steering belts also. The sneaky little gotta that bites most folks is there is a hidden 13mm bolt on the back side of the AC compressor (hidden under the back bracket). That being tight will cause you to have issues getting the AC belt tight. Once you find that sucker and get it loose it is a snap. That belt used to be a personal nemesis until the BB mentioned the "hidden nut".

If you do make a PUP or Junk Yard run, go ahead and get the ones off the AC or power steering also. Never hurts to have an extra you know.

Regards, Good Luck, and may the squeak not be with you.

Paul