That is a little dependant on the size/weight of the blade, I have used 2 - 2.5mm German half tan for scandi sheaths where the knives are small and light, but have gone upto about 6mm on some of the larger stuff. If it is F1 or H1 sized I would go for about 3.5mm (I currently am using 3.2mm).
Thread, if you are using linen go for a minimum of 18/3 for sheaths, 15/4 is better, but more difficult to get hold of. If you are after Synthetic go for tiger thread IMO it's the best synthetic you can buy. For the sheath use about 5 stitches per inch.
As SingTeck said chrome tanned hide of one form or another would be good for drawstring pouches, it may also be worth looking into getting pigskin @ around 1mm as that should also do the job.
Thread, I would go for 25/3 linen thread and do either 6 or seven stitches per inch (more likely 7). All of that is available at LePrevo apart from tiger thread and 15/4.
If you are buying linen thread and it is not waxed then you may require either beeswax or boot polish (thanks for that one Toddy
). The wax both helps to lubricate the thread for when you are stitching and also waterproofs and protects the thread.