I was in a vape store a few weeks ago and mentioned juice > 24mg. He told me they "could not sell it", that it was either illegal or "against regulations" or something like that. I questioned him, but he could not state precisely what law or regulation they would be breaking. He finally admitted it might exceed some "guideline" he could not specify. Interesting. LOL.
Your best bet is to get a bottle of 100mg concentrated nic and add it to your juice. Assuming you don't just save money and go fully DIY. The rough back of the envelope formula is pretty simple... each 0.1ml of 100mg concentrate will raise 10ml of juice 1mg in strength. So if you want to go from 24 to 32mg you add 0.6ml to 10ml of juice or 0.9ml of 100mg concentrate to 15ml of juice, and so on.
(the accountants here will note that my back of the envelope formula will result in a slightly lower concentration and it is better to build a spreadsheet or use a juice calculator. But my method gets you into a safe ballpark, slightly underestimating the 100mg addition)
You also want to buy a handful of 1ml (1CC) syringes. For adding nic I prefer the style that has a slip fit nose, *not* a full "luer lock" and I do NOT use a blunt tip needle. The reason is that concentrated nic is thick, it needs a large bore blunt needle (about 14ga) and that needle holds a bit of nic, not accounted for in the syringe graduations. If you are adding 100mg nic to a 15 or 30ml bottle you may only be adding a few tenths of a ml and what is in that blunt tip could be an appreciable amount (0.1 or more ml). So I think it is best not to use a blunt tip needle at all. A luer lock needle will "trap" nic inside the luer lock, adding additional unwanted nic in a different way. The slip fits, without a needle, are probably the most accurate way to do this without becoming a chemist
Concentrated nic is not as safe as water. Nor is it arsenic

. It is somewhere in between. Use nitrile gloves until you have a handle on your tolerance for it touching your skin. Personally I am not bothered at all and do not use gloves but I have a very high tolerance and occasionally drip 50mg juice just for fun. Some get a tingling sensation or feel heat. Clean up carefully especially if you have pets. Don't drink it or use it as a skin care lotion

. It is probably best to store your working supply in a 15ml or 30 ml bottle, and keep the remaining stock in the freezer, in a heavy duty zip lock freezer bag or something similar, just in case you have an ooops and drop it. You don't want to drop a liter of nic, in a glass bottle, on your tile floor

.
There are many other recommendations and precautions for handling concentrated nic in the pinned topics of the DIY subforum under the eLiquid forum. The above are just my personal guidelines and off the cuff suggestions of the things most important to me.