Check locally for pricing on available materials. We have used a lot of “road mix” which is a kind of gravel, sand, clay, for filling over dirt to make good driving surfaces. If you have a LOT of water and it turns dirt to mud, seasonally, you will want to lay geotextile fabric down first, put your topping dirt on it so you don’t lose the new stuff into the mud.
We bought geotextile fabric in rolls, 14ft wide, which you need to overlap the edges on for a good width, a foot or more, should you want to cover very wide (2 lane) areas of driving or parking spaces. We found it online with a search and ordered it, delivered by semi truck. Even an entire roll is not “that heavy” to unload and move about, roll out. You can cut the stuff with regular scissors or a sharper knife. This fabric lets water thru, but keeps dirt and fill apart, so you keep that firm top layer in place.
We have also used crushed cement and asphalt, as toppings for wet places, parking areas under heavy trucks. Often are cheaper than gravel or crushed stone, work very well. At times you might get a break on the price, should you want a lot, keeps the drivers in work. Summer is better than spring, with road weight restrictions on you pay more for each load in getting smaller loads.
ALWAYS buy the entire truck FULL, even if you don’t need it NOW. Cheaper that way, since your cost is mostly the driver and fuel. Dirt or fill doesn’t spoil, you just put it in a pile out of the way for now. Could even put it on an old tarp, to pick up easier later. You WILL need the fill sometime, and have it already handy, can get the job done now!!