Most tractor lines are steel or stainless steel. Grainger, and Zoro both list high pressure stainless tubing. I am sure that there are other sources as well. A common set up for hydraulic lines is to use a swagelok style fitting, or to flare the line and use a JIC or SAE fitting. I will use Parker examples, but there are many other manufactures out there. For example the Parker Ferulok is the bite type fitting. It is to be used with 3/8" Steel or Stainless Steel tubing 0.035"-0.095" wall thickness and are good for up to 5,000 PSI for most fittings. The advantage with tube bite is that you can get different thread ends, such as JIC, NPT, ORB, etc. If you want to jump a size or two then you would have the best luck with a tube bite.
Also note that Tubes are measured on the OD, and hoses are measured like pipes on the ID. It is normal to use a hose one size smaller than the tube, ex: 3/8" hose with 1/2' tube. 1/4" tube will not let hardly any flow through it and will most likely cause the relief to open on the valve as soon as it is actuated due to the inability to handle the B21 flow.
Another way is to flare the tube, JIC is the most common. You would need a quality flaring set such as a Ridgid 377. For Parker you would put a BTX nut and a TX sleeve, then flare it. For bending 1/2" you would need a bender such as a Ridgid 606, 608, or Imperial 564FH-06 or 564FH-08. Parker also offers benders. The tooling is not cheap for this sort of work. Steel lines look great, however they are $$ to make. I would go with hose unless you think you will use the tools again.
I ended up using all hose lines from Surplus Center. Even with a few adapters and hose routing clamps, it was still cheaper and just as functional.