There are a bunch of variables with light bulbs.

MUST Match

• Voltage - 12V (typical low-voltage lighting), 120V (standard US/Canada), 240V (typical for most other places around the world). Some are actually able to handle a range of voltage, but typically within a range of something like 110V - 250V or similar. Basically there is low (< 50) and high (> 50) and never the twain shall meet.
• Base Type - Edison (screw) bases come in several sizes, though in some cases there are adapters from one size to another. There are also various bi-pin, bayonet, twist & lock (sort of a fancy bi-pin) and others.

Must be in an allowable range

• Power - Measured in Watts. Alternatively sometimes specified in VA (which is sort of the same, it gets complicated) or Amperes (which basically multiples with the voltage to give you W or VA). Within a given category (LED, Fluorescent, Incandescent, Halogen, etc.) more power = more light. But there are dramatic differences between types, with LED generally the most energy-efficient. What you mainly need to watch for is a maximum for the fixture. If a fixture says "Max. 10W" then you can swap a 4W LED for a 10W LED but can't swap it for a 25W incandescent.

User Choice

• Lumens - Unfortunately not always so easy to find. More lumens = more light. This is particularly useful for comparison when changing lighting types (LED, incandescent, etc.)
• Color Temperature - This is the 4000K vs. 6500K number and affects how things look independent of the total amount of light (lumens) and how much power is used.

In general you need to:

• Match the voltage and base type
• Keep the power similar unless you (a) are changing type (LED, incandescent, etc.) or (b) want more or less light (though less can usually be handled via a dimmer)
• Match the lumens unless you want more or less light
• Match the color temperature unless you want to change it to a different temperature

For comparing color temperature, nothing beats going into a store that has a display of different bulbs where you can actually see the light. Due to the way computer monitors work, comparing on-screen is just not the same.

But if you can't find a specific combination (e.g., 3W dimmable LED, 4000K, 12V) locally then a good online retailer can help. I highly recommend https://1000bulbs.com (No connection except as a satisfied customer.) Their web site allows for selecting by type (LED), wattage, voltage, base type, color temperature and more. I don't recommend using a big generic online retailer as they tend to have a very wide range of products with misinformation (since the retailer doesn't really "know" lighting specifically), widely varying product quality, etc.