Via npm:
npm install --save-dev https://github.com/machinomy/types-ethereumjs-units
or yarn:
yarn add https://github.com/machinomy/types-ethereumjs-units -D
Don't forget to manually include index.d.ts within an include section of your tsconfig.json
"include": [
...
"./node_modules/@machinomy/types-ethereumjs-units/index.d.ts"
]