The mattress is going to have to fit the daybed, which is probably a standard size if you buy it online. You can, however, have a daybed custom made, and a custom mattress made to fit.

Try Amazon and EBay for starters.

first where in the world are you???

second try kijiji, or a second hand store...

yard sales or garage sales....or estate sales.....second hand can be awesome...clean it up sand it lightly and apply some spray paint in the colour of your choice.

Explain in more detail please.

I'm getting a new bed. I want a daybed but I don't want to buy the mattress with it because I'd like to have it fit to me. Where do they sell just daybed frames that I can look at online? Thanks