A leak down tester requires a air compressor to use it and some do not have that ability. It will also confirm a blown head gasket by showing bubbles in the radiator that way you can get pump cavitation out of the possible source of bubbles you saw already.
If you do not want to spend the money on a one time use for a leak down tester you can buy a replacement hose for a leak down tester and use it to shoot air into the cylinder. This will not show leak down but will pressurize the cylinder so you can see if bubbles appear in the radiator.
While a compression test will show what cylinder is bad it does not narrow down the possible causes. This is where a leak down test comes in. Applying air to the cylinder lets you know if it is a head gasket (bubbles in radiator) an exhaust/intake valve (hissing air in tail pipe or intake) or a bad piston/rings (pressure in crankcase)
With all that said I really do not think you need a leak down test. We already know if the compression test shows a bad cylinder it is most likely going to be a head gasket or a cracked head because bubbles in radiator have been seen.