Nown again you find them on the UK E-bay as a MA/V Mercury Lamp, however you may be lucky to buy one for about £10 GBP (About $14 USD) but realistically they end up selling for as much as £80 plus ($100USD Plus) and there is the Postage to be added.
Underrunning a Discharge lamp is OK to give a quick fire test but Idally you need to run a Discharge Lamp at correct Current to stablise the arc, especially vintage Sodium Lamps etc as if they are not lit for at least 1/2 an hour plus you may discover when you light the lamp it will not come on as the gas is absorbed by the Glass wall of the arctube.
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Underrunning a Discharge lamp is OK to give a quick fire test but Idally you need to run a Discharge Lamp at correct Current to stablise the arc, especially vintage Sodium Lamps etc as if they are not lit for at least 1/2 an hour plus you may discover when you light the lamp it will not come on as the gas is absorbed by the Glass wall of the arctube.
All the Best
Colin
Alternatively you can use a M59 Metal Halide Ballast without the Electronic Ignitor