I'm sure that there are great prices available to professionals who purchase splines in great quantities, but for the DIYer, this is the option they face:
http://www.lowes.com/lowes/lkn?action=productDetail&productId=231913-972-Q10
Assuming that Lowes's description is correct (perhaps not), with 12 4-foot splines per $116.42 box, this equates to $2.77/lf! Regardless, it is unlikely that a single job would require a $116 box of splines!
One could rip splines from flooring boards--a rather expensive option too.
The option here takes a piece of trim already close enough to required slip-tongue width so that only a single table saw rip is required to produce oak splines that are a fraction of these available costs.
Besides, this method provides the option of ripping splines up to fourteen-foot lengths! I choose seven-footers because they are easier to handle. I can rip what I need and no more.