What do you call this part of the stair?

[aaronfude]

Please lood at this picture:

'm talking about the flat part with curvy sides. I want to replace them and give them a naturatl finish.

So my questions are:

1. What is this part called?
2. Where should I look to purchase them?
3. Are there standard sizes?

Thanks!

Aaron Fude

[Grandpa]

1. Tread Bracket
2. Yes, but they may not fit.

[Buddy Matlosz]

They'd be easy enough to make if you have a bandsaw or scroll saw.

B.

[Joe Bemier]

Are those wood - In older times this work was sometimes done in plaster. The pic looks a bit so.

?