<p>I know postgres has a datatype for storing geographical coordinates. But I'm looking for a RDBMS agnostic solution. Currently I'm using Decimal(25,20) in MySQL. I may be using this data to lookup these locations based on a given distance from a given location later. Which would be the best approach to store this kind of data?</p>

The standard is
here
. Although it's too much for a simple use case such as yours, it may give you some ideas about why it may actually be best to go and use some OGC compliant packages many databases have nowadays, even
MySQL
.
Failing that, and assuming you will implement the algorithms for distance calculations, any floating point number that has the precision you need will work.