According to Does the The AMD FirePro™ W4100 come with both mounting brackets? Regular desktop one and the low profile one? the card is supposed to come with both brackets, so the fault lies with the manufacturer, in this case Sapphire, for not including all parts.
Where did you purchase these cards from ?
The retail version always includes the brackets in the box.
The standard bracket on a W4100 is a high profile and a low profile bracket is always included in the box.