Seeds are illegal in most places, although alot of online seed stores are fake or scams there are some that to produce and deliver some quality seeds, use some of the more established ones like worldwideseeds.com or buydutchseeds.com

seed companies realize that in most cases the person buying the seeds is doing so against the law, so for this reason they discreetly ship their products, you have to remember the business won't last long if no one was getting their seeds, so this is a must for both seller and consumer.

as for just grabbing weed seeds from a bag, well you could do that BUT if you are going to grow then why not grow the best quality, also you will notice that higher quality seeds are somewhat easier to grow and work with.

if enough soil is pprovided you won't have to replace this, unless for transplanting to a bigger pot, just keep the nutrients up, but don't over feed.

as for some strains to reccomend, i am assuming you are newish to growing e.t.c so some easyish srains is what you would be wanting

something like Mazar or Oasis, in the end the strain you choose ultimately depends on your expectations of the high, and other factors like do you want yeild size over smoke quality e.t.c

good luck with your seeds.