For me, a blunt 18ga needle works the best. The smaller (20ga) needles make it difficult to load the syringe, especially with thicker juices.
I use the genuine Joye 510 cartos, so a syringe is pretty much a necessity. With Boge style cartos, I use a syringe for the first fill to insure everything is saturated. After that, it's probably better to drip fill. Down the inside along the metal, not in the center where the hole is.
Many vendors that sell juice and hardware also sell syringes and needles. A few I know of off the top of my head are Litecigusa, Madvapes and Freedomsmokeusa.
Depending what state or city you live in, you may or may not be able to buy the needles and syringes at a pharmacy. I know they look at you kinda funny if you even ask at Walgreens here in TX. It's not illegal unless you use them for something illegal, LOL. Well, thats how the law reads.
So, what type cartos are you refilling?