The DotaLoot database has 611 items for Invoker, prices from $0.03. Rarities: Uncommon, Rare, Mythical, Immortal. Any of them can be won from DotaLoot cases or claimed for free in the daily giveaways — noticeably cheaper than buying on the market.
You can get Invoker items for free in the daily DotaLoot giveaways. The fastest way is opening cases — noticeably cheaper than buying on the market.
The cheapest Invoker item costs $0.03. Prices update based on the Steam market.