220->12 AC voltage transformer into 120 -> 12-20 AC
Based on the information you have supplied, we can conclude that the amplifier has an internal bridge rectifier. The reason you are directed not to use a DC source is because that would concentrate all the current in only half of the rectifier, potentially overloading it.

You could try supplying DC to the battery terminals, but that might introduce noise from the power supply/line that the battery side lacks as much filtering for, resulting in more audible noise.



Your best choice is to get an AC adapter with the right voltages. This doesn't mean that you have to buy from the manufacturer of the amplifier, though. This type of adapter is literally just a transformer in a nice case, and given that you have the specs (14 VAC, 200 mA) that is all you need to match.


If you have a local electronic parts store (new or used/surplus) it is worth checking with them, and if they don't have it they might know where else to look.
It is also possible that you could find a local seller of similar equipment that could supply the power adapter. For example, I'm in the US and I could get a matching adapter (except for the plug which is 2.5 mm I.D.) from Schiit Audio, for (currency converted for comparison purposes) 22 CAD.
Other usual sources for generic cheap stuff including the direct-from-China ones.
It looks like 16 VAC transformers meant to be hardwired are a fairly common item you might be able to find at hardware stores. I would recommend this budget option only if you are confident in doing line voltage wiring and could build the transformer into a suitable case to protect the splices that would be required — and it probably adds up to more in total than getting the proper transformer shipped given the transformer, case, AC plug, and DC plug, unless you have access to an extensive junk-box.