If SeedLaunch Pro™ is sold out and you can't find a starter fertilizer from the professional turf industry, you'll likely have to use a big box store starter fertilizer.
Big-box store starter fertilizers are all pretty similar and you can't really go “wrong” choosing one over another—provided you stick to the recommended application rates. In terms of when to apply, we recommend waiting until after your new seed has germinated as these products usually contain high amounts of fast-release nitrogen, which can cause a surge of top-growth in your existing lawn while you wait for your seed to germinate. The reason this isn't ideal is because the seedlings need to compete with the existing grass for light and nutrients. Waiting until after germination to put down fertilizers with fast-release nitrogen (most big-box) ensures you're not fuelling the existing lawn before your seedlings have emerged.