G'day Simon,
Welcome to the forum, congrats on your purchase of 8SE, in answer to your power questions
1. Alkaline batteries are the best value. Rechargeable batteries don't have the power, they are good for 1.2 volts each (8 x 1.2 = 9.6v)
2. Powerpacks are fine, I use a SCA 1900 Amp Jump Starter and it gives me a couple of nights of power without problems.
3. The computer power pack you show the link to WILL KILL your scope, its lowest power output is 15V.
You can get a power cord at Jaycar or similar places just make sure that centre pin is 2.1 mm not 2.5, it should not cost you more than $15.
On this forum there are many helpful people and a great deal of info, do not be afraid of asking even the most basic of question there will be someone who will help.
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Cheers Keith.