You have lots of options but unfortunately none are cheap. Popular choices are the Canon 100-400 L IS, 400 f5.6 L, Sigma 50-500 OS or 150-500 OS.
They all require very good technique to get good results unless used on a strong (& stable) tripod.
State your budget:
The more $$$ spent, the better image quality..
its a give and take...
Ouch. I hate to break it to you, but the solution to your need for more "reach" will require some sacrifice/comprimise/money.
1.) You can't use your existing lens with a tele-extender from Canon. Some 3rd party TC's might work, like Kenko, but the image quality would be destroyed on a lens like 75-300.
2.) You are already experiencing the benefits of the crop body 1.6x telephoto.
3.) A good quality telephoto longer than 300mm will cost you. Dearly. Like $1200.00 for a 400mm f/5.6 at cheapest. From there it jumps immediately up to $6, $7, $10 thousand dollars!
You might want to pick up a "supertelephoto" camera. Canon has a SX 50 camera that has the equivalent of a 1,200 mm lens, which will get you right up there for sure. It does not let you change lenses; you must live with the built-in lens. It will work well enough in good light, though it would struggle in lower light compared to what you could do with your T1i and the right bright lens.
You would probably want to keep both cameras!!
But that whole SX 50 camera is only about $420.00!
Good luck! Hope you let us know what you end up doing.