I've done it oth ways - E6 to C41 and C41 to E6. By far E6 to C41 is the best for me, since it produces the saturated contrasty images one associates with cross processing.
One tip is that you have to over-expose your transparency by 2 stops before cross processing it. Another is that different films may react differently - that is Fuji Provia 100F may be different than Ektachrome 100G.
As for labs, I know only of one in Boston that I have used a long time ago...
Others in Boston that I know of are:
I don't know any in RI.