We've run into the /dev/random problem when doing heavy SSL traffic. We had to pipe in sources of entropy. It was a janky solution, but wasn't as janky as it might sound.
If you want more than that in a cheap Heath Robinson manner (Americans: think Rube Goldberg if you are unaware of Heath, their work came from very similar inspirations) then many SoC solutions have a built-in RNG of sufficient quality for general cryptographic use and some solutions expose them easily. If you enable the relevant module a Raspberry Pi can provide up to ~550,000 bits/sec when asked to, for instance, which if one-off cost is a factor beats paying a few hundred dollars or more for a USB device providing the same sort of rate.
Based on the HAVEGE research http://www.irisa.fr/caps/projects/hipsor/