Solid core power cords under 300$ - List of manufacturers ?

Long time ago, I had a conversation with Mr. Chang of Lightspeed. 

His opinion was romex was the best. Makes sense when considering that's what is (often) in the wall, which he also mentioned. 

I made many, worked great. Sometimes a little improvement, sometimes none, but never seemed worse. Not like having a PLC, with often is major improvement. 

Not saying here that fancy power cords can't make an improvement. Naturally, once a plug in in the chain, it changes anyway. Just made more sense to me to spend more on the PLC side and use romex than spend on the cord.

Regarding the safety of using romex:

Solid wire made for electricity can take bending, and we DO bend it quite a bit installing it when we build, but obviously, it eventually would break. Wouldn't use it for say, a guitar amp that gets constantly plugged and unplugged. When in doubt, make a new one. 

Fire danger comes from the terminations-that's where the heat will be if and when they are overloaded. Insulation won't save  you or change that, but the quality or suitability of the connectors will. Using the right connectors will make it so any fire or heat will be in the plugs and NOT in the connectors. And the right connectors should say what gauge they are intended for. 

Thus, while making a cord isn't proper code, you have essentially made your own electricity built to code. the risk may be the same, but it's now YOUR risk. 

Just a reminder.
The OP said nothing about using Romex cable.

Solid core power cords under 300$ - List of manufacturers ?

I would like to try a 2nd hand solid core power cord under 300$

What are my options?

I know following manufacturers:

Audioquest (Copper)
Cabledyne (Silver)
Nordost (Silver plated copper)

DIY is also an option for me.
(my devices draw 15 watts only (1 streamer, 2 mono blocks ))

None of the above manufacturers use Romex cable in the manufacture of their power cords. None of the above manufacturers use a solid core wire larger than #18awg. Many use a combination of LITZ small diameter awg solid core wires bundled together under an insulated covering combined with a few #18 solid core insulated wire/conductors.
Example. Look at the picture of the cable.
http://www.audioadvisor.com/prodinfo.asp?number=PGAC14XL2 .

DIY was brought up as an option, and romex was also brought up by at least one other...mainly, because romex IS an option. And definitely solid-core. 

As far as that goes, options would also be buying AC wire (same as in romex, just not in the extra bundling sheath), or, even stranded. Seems bulk was also brought up/used in this thread as well. 

To the point, buying bulk isn't just cheap, but one could easily make the same cable in stranded or solid and see if they hear a difference. 

One might experiment with different plugs as well.