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Most cb750 cafe racers use a stock frame. The stock frame is pretty good to start and was good enough for Dick Mann to win Daytona on it in 1969.
There are plenty of aftermarket companies that made frames for the cb750 back in the day (rickman, dunstall, bimota, seely, etc) those frames are hard to find these days and usually when you find them are expensive. Dresda and a few other frame manufacturers today can make you a frame but it too will be very expensive. Your best bet is to use the stock frame since so much in the way of bodywork is made to fit the stock frame.