depends, lots of seed that you get in your bud aren't going to germinate for various reasons... but i thinnk in most countries its legal to buy seeds (on the net there are loads of sites that sell them) as long as they havn't germinated, maybe not in the USA tho, i'm not sure....
You can definately find viable seeds in your bag, but these dont necessarily have the same genetics as the buds.
The best sources are reliable seed vendors.
If its your first grow I would go with bag seeds till ya get some experience, then go for the good seeds. I'm doing my first grow this year, indoor, and its gonna be sweet.
it's possible, assuming you have the brown seeds (the little green ones are not viable). There are hundreds if not thousands of online seed vendors (seedbanks).