Local racket re-stringers:
A typical squash racket can use almost thirty feet or nine meters of high tech string! It is tough stuff but does occasionally break and even if not used a lot the string will eventually lose elasticity and therefore no longer have the appropriate tension or taughtness.
Each racket has a recommended string tension (24 to 30 lbs) but it can be adjusted a bit, with higher tension you may have a bit more control, with lower tension you may have a bit more power.
Strings are available in several different gauges or thicknesses, it is safest to specify what thickness you want in millimeters as gauge sizes can vary (the US uses a totally different gauge specification). Thinner strings can provide more control but at the expense of more frequent breakages, thicker strings should be more durable but may feel less responsive.
Strings are constructed in many different ways, multifilament is allegedly best if you suffer from arm problems like tennis elbow. You can also get strings in different colours if you like!
Strings are available from several makers including names such as Ashaway, Dunlop, Karakal, Prince and more. Most racket manufacturers also supply strings, so a new racket is likely to have string from the same manufacturer. Unless you actually want to try something different it is advisable to replace a broken string with the same type or something similar in make, construction, thickness and tension.
Your re-stringer won't have every make, type and thickness of string in stock and will normally use their own preferred string and the standard tension for your racket unless you request otherwise. It's not just the qualities of the string that's important, the stringer and the machine they use also affects the quality of the result.
Why not ask your stringer to put a new grip on your racket while it is being re-strung?