If you're using your mobile phone (Samsung or Apple, especially) then those emojis should be in your keyboard, in the emoji option. Emojis are usually grouped into categories (smileys, food, animals) and the zodiac signs should be in the final category, the "miscellaneous" emojis or the "symbols/glyphs" emojis.
If you're using a Windows 10 PC, you can use emojis in the on-screen keyboard that you can click to pop out when it's in Tablet mode. Go to the final category of emojis as well.
Or, you can visit getemoji.com and scroll to the symbols section and just copy and paste your desired zodiac sign emoji. Voila
Hope I helped!
Note: The zodiac sign emojis are usually purple in Samsung and iOS devices, and Windows 10 devices (maybe Windows 8 too). In devices that don't support zodiac sign emojis, they usually appear as typical black zodiac sign glyphs. They'll get colourful again once you post them in the forum, because that's how the forum software renders the zodiac emojis.