Just shop around. As far as I know they're all the same regardless of manufacturer, just make sure you get one with a zoom.
There are a few different styles of ACOG as well, some have the fake fibre optic piece, some have back up iron sights on the top and some have mini-red dot 'docter sights' on them instead.
I have an ACM on my L85 and I've got another for sale, if you're interested? I also have another ACOG, unsure of the manufacturer, for sale too.
There's a slight crack in the front lens of the ACM one and the vertical adjustment turret doesn't seem to work for some reason... But you're welcome to it for £50 - postage included in the price. If you pay the fees. It's in the style of the ones with the fibre optic piece.
The other one is pristine, no cracks, adjusts fine, but it's slightly misty on the inside for some reason... Kinda looks like you're peering through fog when you look through it. This one doesn't have the fibre optic piece, but it has some iron sights on the top instead. It's also with a friend at the moment, might be a while before I see him again to get it back. I'll do it for the same as above, £50 including postage, if you pay the Paypal fees.
Let me know if you're interested.