If you truly have no budget and really, really want to have cleavage, I suggest silicone breast forms that transwomen use. It's pricey at a starting price of $500 or so, but that's the best. Much like prosthesis, they are glued into place and the edges blended out to match the rest of the skin.
An expensive but wonderful example: http://www.transgendered.net/RealDollonepiece.html
There might be a cheaper alternative if you go for the lower quality options, or if you want to make it yourself.
If that's well beyond budget, the next best thing you could do is use lifts to push your breast tissue together, and use makeup to contour, contour, contour.
If you could post a reference image of the corset we might be able to give more advice.