You say "both sides" need cutting down - presumably you mean one end and the back - to do that the countertop needs to be free so they can get to the edges to be cut, and unless this is a big countertop probably cheaper by far (assuming normal thickness countertop) to clamp on a scrap of plywood or such and transport it yourself to a countertop distributor who also fabricates countertops (many do) or a countertop wholesaler who will agree (in advance) to cut it for a price.
Otherwise - a Countertop contractor is your Search the Lisst category for a contractor to do that. A rare few might field cut it, but not many because field cuts tend not to be a straight as one wants for joints and wall confirmation - so they tend to do pretty much all cuts (as opposed to edge routing) in the shop.
Of course, may not need a diamond saw either - if resin synthetic marble (the usual kind) rather than quartz countertop, just a normal fine carbide blade can cut it, meaning a handyman might be able to do it - but whether he will do an accurate cut or not might be another matter - I would go with a Countertop contractor, even though you will likely see a minimum charge of probably about $200-250. Also - if a resin countertop - you do NOT want them to cut it inside the house - odor can last for weeks.
