Check uk.futureelectronics.com
You'll want passives->aluminium electrolytic capacitors, then drill down to the caps you want with the search bar. This is where I bought my caps from, there is no minimum order quantity.
edit: I've checked on that site using the dimensions given in post #17. They don't do 820uF 25V caps in 10mm diameter, only 12.5. However looking at the photo in #17 you should still be able to fit them side by side if you get 12.5mm ones, they just won't be flush with the board as you'll need them to standoff so you can bend the legs into the solder holes. Failing that just lengthen the legs on the one marked C110 and lay it flat out of the way.