I think these are the one's AZBigJohn is talking about, I also use them and find them pretty handy. Reef Culture always seems to have a bunch in stock near the testing supplies.
You mentioned transporting water though so I feel it's my duty to recommend your very first system upgrade be a new/refurbished spectrapure 4 stage ro/di (no flush needed) and a handheld TDS meter. A 5 gallon bucket, heater, plastic spoon, hydrometer (if you don't have one already) and a bucket of salt. This sounds like a lot but it's easy to hook up to a sink and make ro/di and nsw on demand for your tank. If you have a nano it will last you a long time before maintenance.