If you run out of rain water, you should be able to find an emergency stash of distilled or RO water at a shop near you. You could also think about installing a RO filter at home; firstrays sells them very economically in the USA (mine cost a small fortune here in South Africa, but I was quite pedantic about the specs - waiting for a plumber to install a T so I can get it working...!).
Most laboratory supply companies will sell pH and TDS meters; Hanna make some nice fairly accurate and fairly affordable units. It's a good idea to get some
calibration solutions too if you want to maintain accuracy. I have a
combined TDS/pH unit.
You consider if you belong to an orchid society, the society could purchase one and keep it calibrated and test member water samples at meetings as a service. Having your own is handy though.