Speaking from experience from a Simple Stories user...I'd start buying and stocking up now! They are a relatively new company and just a few months ago they already started discontinuing some of their first lines they introduced--including the one that I need to replicate an album I made for myself so I have one to give to my Dad. I've been lucky and can still find most of what I need (from the 100 Days of Summer line), but there are a few sheets that I can't find any extras of.
But in regards to lines/companies in general, even if a company doesn't discontinue a line some stores don't reorder all or parts of a line because there's always something new and wonderful coming out that they need to make room (and inventory $$) for.
So really, if you know you like it and will need it, I say get what you can while you still can.