Not a good idea to use mulch or anything else that can decompose as the bottom layer. Those kinds of materials will soon decompose and maybe even turn anaerobic underneath the soil.

I'd use a coarse gravel, I guess.

I would shovel down 15" into hill on the high side instead of raising that side. So , your low side will be raised just 15" and your high side 0". In this case , everything gonna be leveled without extra materials.

Is your slope South facing? Consider terracing your beds, where they would be below grade on the uphill side, but above grade on the downhill side. Just an idea.

You could just use any soil from anywhere on your property. It will be buried so deep, the weed seeds won't really matter.

If you build any more beds, I would recommend that you level the area by digging and moving soil and tamping it down FIRST. Then install your bed on a level pad. Don't tamp down too much. because the roots may end up in the native soil. Just a little so that it doesn't settle immediately when it gets wet.

If you have gophers, you should put hardware cloth (wire mesh) on the bottom of the bed. Also, you should make sure the walls of the bed stick up a straight vertical two inches or more all around the bed to prevent the gophers from coming in on the surface.

Best of luck to you!

--McKenzie

I second what DirtandYarn said : TERRACE.

In landscape terms it's called cut and fill. You cut out the uphill side and fill it into the downhill side. It's a well established principal for hills. You can use it for one plant or for a whole terrace.