Where can we go to get Granite countertops other than Home Depot?
Much depends upon the size of your countertop and what's available on STX. Smaller projects can be fabricated on island with available stock and previous posts on this topic indicate the price is similar to the mainland (at least prior to Maria).
Most larger projects will be fabricated in FL and shipped to STX by Armery, Home Depot or Italia. The price for large countertop projects is about double that on the mainland (includes product, fabrication, shipping, insurance, handling, installation).
You also can buy slabs in Miami and ship to STX yourself - but then the breakage risk becomes yours and you still may not be able to get it installed once your countertop make it to the island. You are dealing with big, heavy pieces of rock upwards to 1,000 pounds.
Italia is by the refinery. I know they are anticipating - or have just received - a container of slabs including at least one white quartz slab. They also are making concrete countertops to order, which can be an interesting solution.
The local STX counter dealers (Armery and Italia) have varying relationships with suppliers in FL and may or may not be able to get the product you want if you end up having to order. Flexibility and patience are key.