Hi!
Sand in axolotl tanks is still a hotly debated issue!
The opponents of sand in the axolotl tank would argue that all sand is unsafe. Your axolotl will swallow it, and this could of course lead to impactions.
This is true, but usually sand is small enough to pass through the axolotl without causing any trouble.
Because of this, you'll want to have sand that is as smooth as possible.
Most construction sand is jagged (and thus, sharp) because it makes for better concrete. Axolotl stomachs don't like it, though.
"Play sand" is designed for use in children's play areas (playgrounds, sand boxes). Because parents tend to be as paranoid about sand as axolotl owners, it's specifically selected to be smooth and safe. This makes it safe for aquarium pets, but it has its drawbacks as well. Being smooth and small, it's more likely to cloud your water (and clog your filter), and it also settles into a much denser substrate with less pockets of water/aeration - meaning it gets much dirtier much quicker.
So, "play sand" is safer, but requires more maintenance.
Aquarium stores often offer special "aquarium sand", which is either very good for your aquarium, or very bad, depending on the sand and the aquarium store. Always make sure it's a neutral, settled sand, with no extra calcium or stuff like that.
Products called "white beach sand" or "volcanic sand" or "mangrove sand" tend to contain all kinds of minerals. Great for certain aquariums, but not the best for an axolotl aquarium.
It is possible to buy some very good, neutral "aquarium sand". If you have a pet/aquarium store nearby, ask around! It comes with a price tag, though.
Gathering your own sand is not advised. I guess if you have a river or creek nearby, river sand would work well, but it would come with so many uncertainties that I wouldn't try.
I know all of the above doesn't really give you an answer. I think all types of sand have their own benefits and costs, and it's important to know which is which.
Also a good question: why did you decide on sand? Why not a bare-bottom tank or large river stones?
(Not saying sand is a bad choice, but it's important that you know why you chose it.)