12v linear power supply

What amp rating do you need?  If it's for a low power device such as a DAC, then you can get away with a 2A power supply.  I think the best you are going to do is keep your eye out for a used SBooster 12-13V model.  They sometimes are sold in the $300 area.

Otherwise, your only alternative is a Teradak or Zerozone power supply on ebay from China.

Most DACs or preamps or small a/v devices will use less than 1 amp, so a 2A linear power supply will be just fine.  If you want to go for a 4A, it will not hurt anything. 

If you have something like a video processor device (such as Lumegan) or a computer type device like Roon or Intel Nuc, then you may want a power supply that can provide more amps.  The Keces P8 that provides 8A is the most powerful linear power supply I know of, but it's $900 retail! lol.

The Swagman is at a similar level to the Teradak and Zerozone power supplies.  They are just fine, but not as good as Sbooster.  I think one of the better supplies is the Farad Super 3, but that is also expensive at around $600 or so.

FYI, the ifi audio iPower Elite is not a linear power supply.  It may be better than your normal switching "wal-wart" power adapters, but it is still a switching power supply.

I have not done any testing for linear power supplies on things such as network music servers or network switches, so I cannot comment on if this makes a difference. However, any digital transport will be improved by a linear power supply, regardless of how well the internal regulation and filtering is. I will say any digital transport because this includes all transport devices both audio and video. If you cannot hear the difference, it is possible that your system is not high enough resolution to reveal this or that your hearing may not be sensitive enough (no insult intended).

On the other hand, for most streamers and bridges that spit out USB to a DAC directly, blocking power noise (most likely on the ground) is the goal.

Finally, lets talk DACs. DACs are largely analog devices and demand very quiet power. Most have internal regulation and filtering to accomplish this. Even if they have a wall wart or a switching supply externally, it feeds internal regulation and filtering. So bear in mind that what you are doing is providing a quieter input, including less ground noise, you hope.

It is much more than just blocking A/C noise. It is how smooth and powerful the current is. One analogy I use is to use air compressors as a comparison to switching/linear power supplies. You can think of a switching power supply as a really small 5 gallon tank air compressor with a small pump motor running at 10,000 RPM. Then use a larger 60 gallon tank with a 5HP motor running at 120 RPM as the analogy for a linear power supply. In the end, what kind of air/power would you get out of the small 5 gallon air compressor?

Actually, no, its the noise. And i'm not sure what you man by AC noise, the noise is (by definition) AC superimposed on DC and is a product of the way Switching mode power supplies work. ( i wont go into that here, look it up)

Yes, I know how switching power supplies work.  They use very small capacitors and a small transformer.  Then use a circuit that injects DC charges onto the post transformer caps thousands or tens of thousands of times per second.  This is different  than linear power supply which charges the main power caps about 60 times per second (or 120 depending on how you look at it).

I have done work with both switching power supplies and linear power supplies.  The switching power supplies suffer from weak bass because of the small caps and transformer.  The only way to get a switching power supply to really "act" like a linear power supply is to place a very large amount of capacitance after the transformer/charging circuit.  Let your ears be the judge.  The lab measurement devices don't tell the whole story.