Welcome glad you are enjoying your starter setup. There are some places that won't go higher than 12 m.g., especially if it's high VG. Others will be more flexible. There are some good online places too. You might want to go to the e-liquid forum to get a sense of vendors people like, it can often be cheaper to buy online.
You can certainly DIY down the road, but I'd focus on getting a stable vape going. Congrats on wanting to quit dipping, that stuff is pretty potent. This is one time where even with a sub0hm setup, I'd say you MAY be able to use higher nic liquid, provided you aren't chain vaping like crazy or getting sx of nic overload. They kind of creep up on you when vaping, as it does take a bit longer to absorb nic through vaping, than with tobacco, although you certainly aren't going to absorb ALL the nic you are vaping, in fact it's usually lower absorption than with tobacco, so there are certainly folks going a bit higher on a sub0hm setup, though typically the VERY high nic setups tend to be more MTL, smaller devices with less wattage, meaning less juice per puff to be absorbed. Just keep in mind the sx of overload-- headaches, nausea, etc. Might want to check out some of the pinned threads in the New Member's Information forum to start getting more information about vaping, there are some very informative pinned threads in there....
Anna