From what I can see your options are to find somewhere close by, find somewhere that will do accept mail in processing, or else develop it yourself.
The last option can be surprisingly cheap, and a decent option if you’re just shooting b&w which is quite easy to develop.
Actually one last option, if none of the above are suitable would be to see if there are any kits for your Diana, which will allow it to accept 35mm film,
I used the ilford guys in england when I started with 120 film,it works out expensive if
you use them a lot,I then mastered developing my own which is quite easy and cheap
you should give it a go yourself