older tiles oftentimes can be difficult to match. Check the big box stores first for them. As they do sell all types of recessed tiles.

Depending on where you live, you may need to do some research on suppliers in your area. Perhaps a lumber store that has been around for a while?

Then your only bet is to go to the local lumber yards as they may have an off shoot brand that may match what you have.

Or get as close to the style you have. Since its a basement, I don't know if anyone would even notice. Or do some kind of pattern with the old and new? Finding the same tiles can be tough. Especially if the existing ones are old

Don't know if these are what you are looking for, but Lowe's website shows some with a recessed edge: Armstrong 24" x 24' Fissured Angled Tegular Ceiling Panel, Item #: 89091. If your existing ones have been there awhile, color isn't likely to match even if it's the same tile- old ones are likely discolored somewhat.

Lynn, you would stand a good chance of getting your times at this place or one like it. http://www.defoorsupply.com/default.asp

Try Tucker Acoustical in Norcross they have a lot of patterns.