Landings and the top step get what's called landing treads, there narrower and have built in drop so it will come out even with a hardwood floor or carpet.
I'd buy unfinish treads and just cut them to the shape of the treads you have now. For one reason unless you have access under the starirs there going to have to be attached from above with finish nails which will mess up a prefinished tread.
To cut treads it's best to use a slidding 12" compound mitre saw so they can be cut in one pass, if there's a step from having to flip it over it's going to show.
Your going to find most often each tread will be slightly differant in length and some even have a slight angle.
Your also going to need a table saw to cut the to width. Most come to wide to adjust for differant stair cases.
Lowes and Home Depot have stock red oak treads and railing kits. On the net there's dovens of web sites selling stair parts, most have install directions.
Your also going to need a set of Forister bits (the biggest one being 1") to bore the holes for the ballisters and the stair bolts to hold the railing together.
This is one job I'd suggest getting a pro to do for you. One missed drilled ballister, not installing to code and it's a redo.