http://www.historyireland.com/early-mod ... n-ireland/
This is the best source I could find on what you're interested in. It describes the armor and clothing of the galloglass, alongside their weapons and other equipment. It provides historical sources to backup its claims.
Osprey Publishing also did a book on them. While I haven't read it myself, judging by their other books it most likely would be a worthwhile purchase if you want to be historically accurate.https://ospreypublishing.com/galloglass-1250-1600-pb
If you're simply looking for a place to purchase your equipment, I use Kult of Athena. While it certainly looks a little sketchy, I've gone through them a number of times for arming wear without trouble. I don't know if they'll have everything for a galloglass, but I know for a fact they have the chainmail and arming wear.http://www.kultofathena.com/
I hope this helps your armor and garb needs.