Roller balls are smooth flowing ink pens. The Pentel Hybrid Gel Roller pens are without a doubt the best smooth flowing ink pens I've ever used. Blue ink Roller ball pens. I've found my favorite brand isn't the Pentel, but is the Pilot P-500 extra fine blue. I wouldn't have thought that a change in ink color would be so related to smoothness of writing, but I suspect that different pigments have different consistencies and flow better in one design than in another.
Fountain pens can also be tough for lefties.
Sheaffer pens are inexpensive, but they write quite nicely, and their fine nibs are a lot finer than other brands. I used to think that Parker was best, but none of my last 3 Parker fountain pens wrote as well as my first one. The three Sheaffers I've purchased have all been great. Sheaffer's blue-black ink isn't quite as black or as smooth as Parker's, but Parker discontinued theirs, so there's no contest. Mattresses and beds.