F.I.Y. ~~~~ not fi-sure if it's still the case,
but a few years ago,
you could get handmade dresses made at the craftmarkets
...i.e., at the Negril-airport & at the downtown craftmarket

the seamstresses had a sewing machine on-site

Once upon a time, the lady at the downtown craft-market made my mom two fabulous sundresses. I took about 1-1/2 days.

In September 2011,
the lady who worked at the Negril airport
was set-up in a shop in the Lucea-market
...dunno if anybody-else has taken her place

if you can't find the lady at the lighthouse,
i suggest asking-around the town
...the odds are pretty-good that there're several likkle-shops around Negril

I would go to:::::
(1)....the bus/car-park over-by the police station
and/or
(2)....down the lane/road,
in front of the BurgerKing, around the corner from ScotiaBank