Take it to a local machine shop.

They might be able to machine you a part for under the cost of new ones. Especially if it's just a threaded part or bolt, or both. While $250.00 doesn't go a long way these days it's still a fair amount of money. . The "band" it self would cost too much to fabricate a new one.

These filter bodies can be funny. Try re-seating it a few times. Often times you might be trying to "squeeze" the two body halves too much with the clamp assembly itself. What happens is, if the the "O" ring is compressed entirely via the clamp band it can "roll" and become malformed. Try disassembling it, and when you put the two halves together some how (it's a pain) try pushing them together evenly with some weight. I used to remove the gauge or bleeder on top (as to not damage it) and sort of bearhug it the top half down onto the bottom. Make sure it begins to seat evenly around the circumference of the halves . You can't possibly push it all the way together but make sure it begins to "close" some what before using the band clamp to completely seal it. You might need help, one person applying pressure to the upper body half, and the other putting the clamp assembly on. It's going to want to "pop" up while you putting manual pressure on it.

One other thing. The clamp itself is in the profile of the protrusion where the "O" rings sits on the body. Make sure it didn't get splayed outwards. This can happen from continually using the clamp to pull the body halves together from their most separated positions. Put the filter together with out the O ring in it so that it seats flush, then hold the clamp up to it and make sure it still follows the profile of the protrusion. If not, squeeze it in a vice, carefully and in small increments till the areas that have become "splayed" out ward are more in profile with the protrusion.

There also might be a notch in each segment of the filter halves, in the the area that the "O" ring sits. Make sure these are aligned. For the life of me I don't know why it makes a difference, but sometimes it does.

I'd try to exhaust all possibilities, even just re-seating it again. You might have had the "O" ring become malformed or "rolled" the times you've tried so far, one more time might do it.