I'm pretty sure you won't be able to fit a 5x7in back to your 5x4in camera, but there are quite a few old 5x7 models around.
You can still get the film too. Try Retrophotographic. They have quite a range, including Adox 5x7in. I get my 9x12cm sheets from them. I'm not sure about commercial processors though - I do my own (BW) in the sink.