There are a few different types of angle brackets or L-grips to choose from.
The lightest would be example 'A.'
The heaviest would be example 'B' from Vivitar. The Vivitars allow for interchangeable base brackets.
The most expensive will probably be example 'C' from Mamiya, with a fair amount of weight, too.
Most of the Mamiya grips only allow the flash to be pointed forward.
The Vivitar grips usually have a swivel shoe, in case your flashes don't have swivel heads.
A)...Plain Flash Bracket:
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B)...Vivitar Flash Bracket that accepts a common cable release trigger:
Dead Link Removed
C)...Mamiya C-Series and RB/RZ Grip that fires/activates the chin button:
http://img.auctiva.com/imgdata/1/6/9/3/8/4/6/webimg/608471281_tp.jpg
D)...Vivitar grip, (same as 'B' above) but, with wider base specifically for medium format cameras:
http://thumbs2.ebaystatic.com/m/maX7LuwUlWhfP012Ha4bqlA/140.jpg
E)...Potato Masher flash units with built-in angle brackets,
available from Metz, Sunpack, Vivitar, and other manufacturers:
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Just a few considerations.
If you are new to TLR's, I would start with the simple bracket in example 'A.'
Marc