unicode symbols for entity-relationship
From my experience, simpler characters are more commonly used, e.g. 1-1 to mean one-to-one, and 1-* for one-to-many, and *-* for many-to-many.
You can also use the infinity symbol (U+221E) for the &quot;many&quot; symbol if you really wanted but I prefer the previous simpler examples.
By the way, that website is helpfully searchable and you can find other similar arrow characters to what you're looking for as well such as the following:

Rightwards Two Headed Arrow (U+21A0)
Rightwards Double Arrow (U+21D2)
Rightwards Triple Arrow (U+21DB)
Rightwards Arrow with Double Vertical Stroke (U+21FB)
Rightwards Two-Headed Arrow with Double Vertical Stroke (U+2901)
Rightwards Double Arrow-Tail (U+291C)