[bojan]

I had look at what Steve Mogg has..
It seems he is re-selling the same adapters available on ebay, with his price margin. They are all coming from various manufactures in China, anyway..
He does not have Sigma -> EOS adapter, though.

Because Sigma produces lenses for various cameras (including Canon), I do not think there will be adapter available from third-party vendors, since it is easier to go to Sigma service and ask for conversion.
Perhaps this is what you should try first.
If they can not or do not want to do it for reasonable price (which is quite likely if lens is older), then the modification is certainly an option.
That is, if this particular lens is really that good (which may be the case, but I do not know for sure.. I tried only Sigma 35-70 Zoom lens once, and it proved to be quite sharp, even in corners of the frame - of course, I had to step it down to f4).