Rocks on a slope can be quite stable.
They need to be well dug in, and the soil packed in around them.
If the rocks you want to use are not much larger than about 100 lbs, then a single person in good physical shape could handle the project, though having a strong helper would be really nice.
If you want rocks over 100 lbs a couple of people might handle the job, but on that steep a slope you might want to rent a tractor with a backhoe to help dig the holes, move the rocks, then stabilize them while you backfill. This is still a DIY job, though I would suggest that at least one of the team have some experience in construction, or similar heavy work.
You can build a rock wall at the bottom of the hill. They are fairly easy, just take some time to study how to best place each rock. You do not need to mortar the rock in place. This method is called Dry Stack. you can make retaining walls to 3' high this way.
As you build the wall, backfill with a good soil so you will have a somewhat level area with good soil for planting.
If you want to terrace the hill, start the next wall at least 6' back from the lowest. This is almost impossible with a 40* slope. It works with a 30* slope.
Sources for rocks:
Look locally with these words in the search:
Landscape
Rock
Boulders
Masonry
Brick
Soils
and similar words.
Around here stores sell rocks by the truck load, and individually. If you have a pick up truck you can haul your own rock. These stores also sell brick, bags of concrete, mortar and related materials, bark, mulch, slate, and custom rock materials.
Go in person to see the rock, to be sure you are getting the right material. Different rock will be different colors, and different shapes. Some look very natural. Others stack up quite easily to make a wall.
Look around your neighborhood. If you see some rock work you like ask the homeowner what rock it is and where they got it. You might find out they had it professionally done, and their contractor found it.
