Have a look at Shimano tech docs site, here you can see the official tech sheet detailing bleeding spacer you need. You may get lucky at your local bike shop, if you ask nicely they may have one to spare. If you want to save a few pounds and make one yourself I'll take you through what you need.
The Park Tool site details the thickness of the spacer required - 10mm should do it, now all we need to do is to find something we can use which is the appropriate width. I racked my brain for a while trying to come up with something.
Having spent many an hour inspecting and measuring bits and bobs I came up with using one pence coins - they are the correct diameter and stacking them comes to the required thickness.
It is best to have a selection of ages of 1 pences available as they tend to loose thickness as they wear out. According to the Royal Mint, brand new pennies come in two different sizes, either 1.52mm or 1.65mm thick dependant on the material used.
Be aware though this is not an exact science as I tested six older looking coins against 6 newer ones - the result was 9.20mm vs 10.20mm so there must be another element in there also a bit of dirt and a bit of wearing thin.
This is great, we can use it to tune the tickness of bleed block you want. People rave about Shimano brakes as they aren't draggy. Whilst I admit all of my Avid brakes have a little bit if disc rub most of the time they have an excellent automatic bite point adjustment.
Shimano on the other hand do not have such a feature and as the pad wears you have to keep winding the reach of the lever outwards so the lever doesn't bottom out on the bars so you can actually stop (some higher end Shimano levers do have a manual bite point tweak but this still doesn't give enough adjustment). This isn't a problem initially but it soon makes the levers so far out that I have ungrip the bars to fetch them back in preperation to use them! I'm glad I only have them on XC bike.
I've read somewhere on the internet (I'll supply a linky when I find it again) that as the pad wears down you can use a thinner bleed block to keep on top of the reach issues.
What you'll need:
Glue it together
Then all you need to do is to select the combination of coins you require to make up your disired thickness (I went with 9.80mm) as measured by your vernier guage, glue them together (I used some E6000 I had lying about after I was gluing my shoe back togetther). Once the glue has set you are away.
I found in use that the spacer did fall out a couple of times, not during critical points of the bleed process though. Other than this it performed admirably and despite it's slight drawback it is great to be able to slightly tweak the exact bleed you are getting out of your calipers - I found that 9.80mm was spot on for my current amount of pad wear.