Where to buy a replacement DC adapter?

Any hobbyists electronics shop (Maplin, Radio Shack, et al.) will have these. Just take the faulty one down and ask the staff to find you a replacement.

If you insist on buying online, triple-check that it's DC output, not AC. Also carefully measure the inside and outside diameter of the connector as there are several different popular sizes. Most are positive on the inside but that's another thing to check.

Yes, it's off topic. And Please read the FAQ/Wiki as suggested:

https://www.reddit.com/r/AskElectronics/wiki/faq#wiki_tips_for_choosing_a_replacement_power_supply

Plus, since we don't know where on the planet you live, making good supplier suggestions is tricky. Please also see in the FAQ:

[How to ask a question on r/AskElectronics](https://www.reddit.com/r/AskElectronics/wiki/faq#wiki_how_to_ask_a_question_on_.2Fr.2Faskelectronics)...

• Location - If you are trying to locally-source a part or piece of equipment etc., please state where you live.