Remington makes the brass "seasonally," so if you want actual Reming .17 Fireball brass you buy as much as you can, when you can. Or buy factory ammo and shoot it up. It has shot very well in several rifles for me.
With all the factory rifles I've shot, .221 Fireball brass necked down has worked fine as well. The shoulder angle is slightly different, but if you make sure to size so there's a little snugness when chambering a round that's not problem. The necks don't end up too thick for factory chambers, and you can almost always find some kind of .221 brass, though the price can be a little steep for some brands. Though a quick search found Nosler and Norma .221's for around $60/100.
