also is steel better or chromoly? what is the best material aside from these?
Chromoly is a type of steel. Only the very cheapest steel bikes are made of non-chromoly steel. If you buy a steel rigid fork with disc brake tabs, I doubt if you can find one that isnīt chromoly.
There are many to choose from. Surly, Dimension, Nashbar, and Salsa to name a few.
Kona also maker a decent chromoly steel fork available with disc tabs. http://www.konaworld.com/tech/components.htm
The best steel for forks is chromoly steel that has a butted profile.
Look for one with threaded eyelets at the bottom to accept proper fenders.
You have to get the correct size and style for your bike.
If you have a threadless headset then you will have to get the fork cut to size. Better to leave it longer than needed and use spacers than accidentally cut it too short and have your handlebars too low.
It is possible to make expensive mistakes fitting forks ; it may be better tell a decent bike shop what you want and let them accept all the risk.
My Kona Jake has a P2 (700c, non-disc) that I like a lot.
My beater MTB has a no-name steel rigid fork from nycbikes.com- heavy but dirt cheap at $19. Can't remember if they have disc tabbed ones. Also Dimension/Tange have cheap forks, look on any site that has the QBP catalog (aebike, triathlete.com, etc.)