The Black-Letter Capital I symbol (β). Copy and paste it, or use its Unicode, HTML, and CSS codes.
More from Letterlike Symbols.
The quickest way is to copy β from this page and paste it anywhere. You can also insert it with its Unicode, HTML, or alt code listed above.
β is U+2111 (decimal 8465).
Use ℑ in HTML to render β on a web page.
On Windows you can type it with Alt+8465 (hold Alt and enter the number on the numeric keypad).
Yes β copying is completely free and unlimited, and works right in your browser with no sign-up.