You can find a multitude of reference styles at zotero.org/styles or by clicking Edit > Preferences > Cite > Styles > Get more styles in Zotero.
It may happen that you cannot find the exact style you are looking tor. Maybe you need a variant of a well-known style or you do not know the name of the style that you want – e.g., when a journal doesn't offer a CSL style, but only a style sheet. In this case, you can try the options below.
You can find a style that is similar to the one you are looking for by searching by appearance on http://editor.citationstyles.org > Search by example.
If the exact style you are looking for does not yet exist, you can request a new style. Look for "Requesting a new style" on zotero.org in order to see the details on how to proceed. You will for instance be asked to provide links to the style guide of the journal and give some example citations.
You can edit a style or create your own. Note that this takes some technical know-how. You must edit the CSL (Citation Style Language) file using XML programming language. More information can be found here. A fairly easy way to do this is by modifying an already existing style similar to what you want.
Use Zotero's Style editor
Use the Visual CSL editor
Go to https://editor.citationstyles.org/. Here, you can either (a) go to Visual editor and choose Load style in order to choose a style that you have saved as CSL file on your computer or (b) identify the style that you want to use as starting point and click Edit style; the style is opened in the Visual CSL editor.