I did the exact same thing about 2 weeks ago dude. They're called spacers...fucking pain in the arse...
I ended up just taking down the remaining one to a specialty diff place and asking the dude if he had one to suit. He went out the back and grabbed one and gave it to me for no charge(needless to say I know where I'll be going to get diffs rebuilt in future...)
When putting the locked diff back in just make sure that the axles are well and truly out of the way or you won't be able to get the diff in at all. I ended up taking them out altogether. If there's even a mm of the axle still obstructing the diff it won't go in. With the axles out the diff should just slide in real easy....like ur mama...
Rory
PS: don't think putting in a spacer that's smaller would be a good idea bro. Freeplay in the diff would be a no-no as far as the teeth are concerned I would think...