The original one-piece ceiling splines have been out of production and out of stock for many years. More recently, Okaply Industries in Quebec, Canada was manufacturing a similar ceiling panel with a two piece plastic spline, which was totally different than the original type splines. I just did a quick search of the Okaply web site, http://www.okaply.com/
, and it appears they are no longer selling these panels. All links and mention to the panels have been removed. There may be some mobile home supply stores that will still have some old stock, stores like mobilehomepartsstore.com, mobilehomeadvantage.com, hsmhs.com, randgsupply.com, etc.
The original spline were installed by squeezing the spline and guiding the edges into horizontal slots cut into the ceiling panels. The Okaply splines were a thicker plastic that clipped into the 2nd part of the trim, which was nailed to trusses, or clipped into vertical grooves in the panel itself.
I repair mobile home ceilings quite often and my stand is that the splines are not available. I have used painted wood screen trim to fake in the splines. With caulk and paint, they look alright, but not a match at all.