My MX-900 Universal Remote Control is completely draining new batteries after just 4 days (with minimal use). The remote is 6 years old and has never had this problem before.

There should be short circuited component/s inside the remote control. It will be good to buy a new one, rather than try to repair this remote control.
If you wish to buy its original remote control, contact the authorized service centre, or authorized dealer to your device brand. They can either give you a genuine remote control to your device, or can direct you, from where can you purchase a suitable one.
To select a proper universal remote control, you should know about the number of digits to its set-up codes. There are 3 digit, 4 digit, 5 digit, 2 digit and even single digit codes, to some brand devices. After confirming this, check the sites linked here. Both the sites have set-up code list and set-up procedure to many popular brand remote controls. Check which brand remote control has set-up code/s to your device brand, and buy that one according to availability? Brand name of the remote control isn't a matter, but it should have set-up codes in its code list, to the device brand you have.
Please note: Every new universal remote control will have set-up code list, and set-up procedure along with its pack. Never accept a remote control, that hasn't both of this. For more details about the set-up codes, visit the site http://electrohelponline.blogspot.com/ Check this site too. http://remotesearch.blogspot.com/ Universal remote controls are available at most of the electronic spare part selling shops. Visit some online sites too, which deals with electronic spare parts.