In my experience the cheapest way to get a generator like this is from a private party. For instance I have a boat anchor generator that goes to 400 MHz, calibrated and leveled, with AM built in, that I don't use. Surely there is someone local to you that has something similar to give or lend or sell to you. PM me if you are near Los Angeles.
I don't think there is a better answer. Of course the heavy tube units are electrical bargains but you need to consider the space they take.
Insidentally I recall seeing a rising output voltage with frequency until I realized the test cable wasn't terminated.