Most coralline 'seeding' products don't actually contain any of the algae, that would be very difficult to preserve long term. Rather, they are a mix of supplements that are tailored to best boost the growth of coralline in your system. I started my tank with mostly dry rock and a few pieces of live, and by now (started last september) about 30% of the rock surface is covered, not counting the the back glass which at one point was near entirely pink. So you could honestly just get a few frags with coralline on them and wait for it to encrust with good enough results, but it would take a while.
My favorite method for spreading coralline is getting pincushion urchins. They try to eat it, but most of what they crack off the rock floats through to water and starts growing somewhere else, essentially doubling or tripling the original range of the algae.