Correct. The ONLY way to get slayer accessories is by using $20 USD to purchase the forge pass. You do also get them for the $50 forge pass but the $30 extra gets you relic crafting parts, gems, and relic upgrades. Most people will tell you though that it isn't worth it though. While the relic crafting ingredence are unique; they're kinda junkie.

You do get a few free by progressing the Forge Pass quests.  Of course they are lower quality and not as numerous.