There are many calcium indicators (a.k.a calcium dye) around. One good place to start is the website of ThermoFisher:

https://www.thermofisher.com/nl/en/home/life-science/cell-analysis/cell-viability-and-regulation/ion-indicators/calcium-indicators.html

I'm more experienced with fluorescent indicators. Depends on your purpose, you will need to choose between ratiometric or non-ratiometric dyes. If you just need a relative comparison of calcium levels, a non-ratiometric dye from the Fluo series such as Fluo-4 would be a good start. If you need to know a proper readout of calcium concentration then you will need to use a ratiometric dye like Fura-2 or combining a non-ratiometric dye like Fluo-4 with another calcium insensitive fluorophore like AlexaFluor 594.

The other aspect you need to consider is the range of calcium concentration ([Ca2+]) you need to measure for accurate measurement. Typically a good rule of thumb is the [Ca2+] should be close to the dissociation constant (Kd) of the dye, e.g. Kd/10 < [Ca2+]< 10 x Kd. The manufacturers usually give the Kd value of the dye.

Lastly, the formulation of the dye is also important. For extracellular you may want to use cell impermeable dyes. Cell permeable versions of the dye (usually acetoxymethyl-ester, a.k.a. AM-ester or AM-dye) can be used to label cytoplasms of a cell without needing to disrupt the membrane. Otherwise, one can introduce the cell impermeable version via a microelectrode/patch pipette.

Ca2+-selective electrodes provide a wide dynamic range but most labs do not have the hardware or expertise to make and use them. Most fluorescent Ca2+ indicators were optimized for measuring intracellular calcium levels and therefore have high affinity for Ca2+ and saturate near extracellular calcium levels. However, there may be some low affinity Ca2+ indicators that may work. Paredes, 2008 reviewed the topic.