Since downhill mountain bikes have "upgraded" to 27.5" and 29" wheels there are many used, dual crown aka triple tree, racing quality forks available in good used condition. The question is what you can afford.
A downhill mountain bike only goes fast for a few minutes, even the longest races in the Swiss alps are only 15 minutes from the top to the bottom. If you are riding at 40mph for extended periods of time you really need lots of oil to absorb and pass on the heat from the damping (ALL kinetic energy "absorbed" in the damping of a shock absorber is turned into heat, only the spring returns the kinetic energy).
The old fashioned coil sprung and open oil bath damped forks (old mx technology) will provide the heat sink and heat transfer you require.
The company that forst brought this tech into the mtb world was Marzocchi. These forks, originally made in Italy but developed in the USA, were for 1/18" threadless and 26" wheel. The quality of these original forks was impeccable. The production moved to Taiwan and the air tech took over due to weight concerns, and the quality went down, now the brand is no longer popular even with the mtb scene. However some old open oil bath forks are still out there and they last just fine if they are serviced and seals are replaced when necessary.
The only recent and "low priced" dual crown fork made with open oil bath, that I know of, is the Chinese made DNM USD 150. I have no idea how good it is.
This is NOT the DNM USD 8 which is a cheap and very flexy Chinese made air sprung fork.
Marzocchi Super Monster T
Marzocchi Monster T
Marzocchi Junior T
DNM USD 150