Looking for the Colt (semi auto) ambidextrous safety selector? Me too... 
The ambi lever is made up of three separate parts that Colt supplies, some vendors assemble them for sale as a whole unit. 
Colt part numbers   
SP400025(ambi handle -right side) 
SP400583(cap screw) 
SP 400831(left handle with semi-auto barrel) or SP400024(left handle with full auto barrel) *DO NOT use this in a semi-auto rifle 

There are a couple of dedicated suppliers who stock these parts 

Arms Unlimited- (Best price) not in stock but check back, more to come 

Brownells- (more expensive) but usually in stock with the following part numbers. 
160-308-425 semi barrel  
160-308-364 Full auto barrel 
160-308-426 screw  
160-308-424mb right side lever  

G&R Tactical lists them but have not seen stock in a while. 

Midwest Gun Works lists them but have not seen stock in a while.

dupe