There are many types of pH glass electrodes. In some specific applications you should be very carefull when selecting one, but in most cases the selection is easy. Look for other users working in similar environment and ask them about their experience with different types and makes of electrodes, that way you should be able to find the best offer pretty fast.

If you are working with aqueous solutions containing at least 5% water and your solutions doesn't contain any substances reacting with silver, look for general purpose electrodes.

If you work with solutions containing organic material, proteins, TRIS buffers, heavy metals, or with very low ionic strength solutions, look for calomel electrode. Listed substances can react with silver and clog the junction.

Instead of using calomel electrode you may look for double junction electrode, as it will have similar properties.

If you are working with solutions that can clog normal electrode junction (like oils, foods or paints) look for teflon junction electrode. Junction in these electrodes is made of porous teflon, making it resistant to impurities.

Don't forget to check electrode pH range - some electrodes can't work in high pH, and electrode temperature range - especially if you are going to measure pH in solutions above 60°C.

In most cases manufacturers sites contain a wealth of information about available pH electrode models and their applications.