What are you trying to achieve with the new lens? Do you want in closer (more magnification) or do you want more working distance around the subject?
Bellows aren't a bad idea, and you can adapt manual lenses to bellows without worrying about infinity focus etc.
You can pick up EF bellows (without communication) or M42 bellows pretty cheaply.
I used to use Minolta SR cameras and had a set of bellows adapted to my 400D with Minoltas very decent f3.5 50mm macro. This could give up to 3x lifesize magnification at the film plane / sensor, and with the crop became effectively more telephoto as well (note difference between crop and magnification, different concepts although similar ends) and you could probably pick all this up online pretty cheap, MF ring, aperture ring. Sharp.
You may also want to consider a sliding macro plate, something like a manfrotto 454, as you'll set the magnification on the bellows, and focus by altering the film plane-subject distance.