Where can I buy one of these ?

[Vandad]

Guys please help! Where can I get the holder clamp thingy which clamps onto the handrail and another metal tube (in our case the A frame) goes into it so effectively the a frame will be mounted onto the handrails?

What's it called?

[Landlockedpirate]

Hinged jaw clamp or slide. Lots more choice on the other side of the pond, but here's a few over here.

bimini jaw clamp | eBay

[Jizm]

www.bainbridgeint.com

[Portholme]

Try Sea Screw