For that little water, just get a API tap water filter.....0 TDS all the time. Cheaper than Ro/Di but doesn't last as long as a ro/di. This might pay for itself with about 300g b4 u need to replace filter
Pop corals has it for $8 for 5 gallons i believe on 67 and 13th ave
What part of BK? There are several aquarium stores and most should have it. Pop Corals and Coral Theory both for sure. There are some others too but are not just salt water and may or may not. I would help you out but I just moved and haven't hooked up my rodi yet I have just been buying it until I do.
Sheepshead. I've been using distilled and i prefer distilled but its alot more expensive.
Same here. Been using distilled for last 5 years and works great.