Insya'allah!
You can drop by your neighbourhood muslim salon/barber to ask them regarding this matter as i think
they are the best people to ask regarding this henna matter.
You can too purchase henna in packet in some
convinient store and mall.It's not that difficult to get it nowadays, remember to check for the expire date before you make purchase ,this is important because it won't be in best condition if it has expired.
Also check out the many youtube's video on henna highlight;search on Henna highlight.
i think you will get some idea on the better application by yourself with henna.