Double check your measurements - I think you actually need a 5.25 to 3.5 adaptor for a card reader.
Assuming that's right, it depends on whether or not you need the bezel. If not, you can buy a pair of brackets like these, otherwise get a complete assembly like this. If you really do need a 2.5" converter, something like this perhaps.
(I'm not recommending those specific products or sellers - you can buy the same things from any PC hardware supplier or through most online stores and auction sites)