NIB (New-in-Box) Intellivoice Units can be had at online auction sites for a fraction of their original value at any online auction site or flea market. In 2022, do not expect to pay more than $25 USD for a working unit with box, less if unboxed and slightly more if shrinkwrapped.
Because Mattel Electronics misread the market in late 1982 (buyers became enamored with arcade ports), and the games were complex and expensive to produce (dedicated months of salary people like voice actors which has never been done before), far more units were produced than sold. Mattel Electronics tried different cost-cutting bundles as well, flooding the market even more.
Voice games will work without the adapter, but since the voice was made to be an integral portion of the game, the original 4 titles are extremely difficult to play without one.