He is looking for 5.7x28 ammo. Your link is a good place to start. It seems that FN has a pretty big order for a foreign military so there may be shortages for about six months or so. Elite Ammunition has some in stock, but it isn't their ammo and not the SS195/SS197 that are in the stores. Their ammo is better, but more expensive.
Up until recently the round was found for a reasonable price on a lot of online ammo sellers. Each round was around the price of a .45 at around 40¢ a piece. I do agree that you should stock up once you find them at a good price.Well, if you choose a round that's difficult to find and also expensive, you should stock up on it when you do find it.