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Likely a special order thing. Midway, it's a good place to find out what's available Stateside is showing Remington ammo as 'out of stock, no back order'. I don't think the only other brands are available here. Graf's, another U.S. site, doesn't list it.
Epp's lists ammo, but that doesn't mean they actually have any. Worth an e-mail though.
Brass can be made by necking down 221 Fireball or by chopping and sizing 223 cases. The caveat being that in both cases you need to experiment with neck turning or reaming in order to get the correct neck wall thickness.
17 Fireball is a very cool cartridge. Loads of fun to shoot.