There are many scenarios that will determine how close you need to match up drives, such as:
- expand a current raid array with additional drives
replace a bad drive
For both of these, the recommendation is going to be stick with the same model drive if at all possible. Although some controllers will be more lenient even letting you use a larger drive and it will just not utilize the full drive.
replace current drives with new ones
- add an additional array of new drives
In the case of these two, you will have a lot of freedom as long as the drives are the proper interface (SAS, SATA, etc). There is a chance of conflict between some controllers and drives, but that is the exception not the rule.
If your server is from someone like Dell, HP, IBM, etc check out their parts websites for your server (there should be a unique identifier on your server so they will have information about your specific machine). Even if you don't purchase it from them, it is a good way to see some of what will work for your system.
For other manufacturers, just start in their support section and look around, you should find replacement/upgrade parts information.