[Link: ebay.com]There are a few emitters like this that are a little more powerful. Mostly came bundled with cable boxes and old TiVo units, now can be found on eBay.
There are 2 ports on the MRF260 that have adjustments. You would plug these into those and turn the level up to max.
How far are you trying to get the IR to reach? There isn't anything you can plugin that is going to be as powerful as the IR blaster built into the front of the MRF260 already.