If you're looking for material, a really good place to look is JSTOR.org, but it's subscription only so I'd check if your school/college/university has a subscription but it's really useful if they do. If not try looking on google books or google scholar for stuff that's been written.
Also, you might want to try this website, it's totally free and it has loads of digitised primary sources and is super useful for pretty much any time period (just google Fordham Sourcebook and it brings up all their sourcebooks by time period) but I did a quick search and this is came up when I typed in celts: http://sourcebooks.fordham.edu/Halsall/sbook1g.asp
Good luck!