I got some puffer type bottles off ebay that worked well for the job, if you search on something like "plastic dropper bottle" you will have more choice than you can imagine and they are inexpensive.
I use two different things in them. One is chalk for marking a chalk line - this comes in big bottles and is inexpensive and you can probably get it from your local hardware store. This is what I have, but with the cost of postage probably best to get it locally:
Faithfull Chalk Line Powder White | eBay
I find that white chalk is good in forestry as you can watch it drift for quite a distance among the trees and the white makes a good contrast against trees in low light. Being able to watch it for a little while is helpful in very low wind conditions as it might move off in one direction, and then change.
If you want something that will carry well on the very lightest of winds then get some lycopodium powder, this is used in special effects etc. for making flames but it is a very fine powder and it makes the wind really well. However the lycopodium is yellow in colour and i find it isn't so easy to see plus with being so fine it seems to disperse quite rapidly. It might be slightly better for very light winds but in general use the white chalk works better in my view.