Where can I purchase a block of aluminium?
There are several online metal suppliers that will also make custom cuts.  Online Metals is one that I have used.  There is a minimum length that you must order (usually 1 ft) and a charge per cut.  Depending on where you live you might also find a local metal supplier that can do the same for you.   

Wherever you order, I'd suggest you get two 5" pieces cut from 1ft of 1" x 5" bar and then you'd have a spare.  I would definitely try to have someone else do the cutting for a piece that size.  Rectangular tubing might also be something to consider.

Here is a reference on the same supplier for the aluminum alloys they stock.  Be sure to check the prices as they'll vary, but I believe 6061 is one of the more widely used alloys.