You really should mix them yourself imo, the bottled ones tend to be pretty sweet, which diminishes the refreshment. Ratio 60:40 beer to lemonade at least. I just tend to use whatever is in the house, might be something like Bitburger, König Pilsner or some other regional pilsner... Beck's should be fine although it's not my favourite.
The pilsner's bitterness tends to fluctuate a lot throughout Germany and adjacent countries, in northern Germany they're way more bitter than the Austrian Stiegl you get, which makes them more refreshing to me in a Radler.

Sorry for the essay on beer and Biermischgetränke :D