When I did my first DE I needed a helmet and didn't have any place local to go so ended up going online. I found it to be completely satisfactory.
If you're not sure of your size, just go to a reseller and read their sizing instructions carefully and measure your head appropriately.
There are many web stores but I found a few (none of which I have any stake in) that seem to have the better deals. Try soloracer.com and or saferacer.com for pretty good selections and prices.
Soloracer has a really good information link that you should look at. It is
SoloRacer.com Helmet Guide
I'll tell you what worked for me and how I got the right fit the first time. When I measured my head, I ended up at one end of the usually quoted range for a given manufacturers size. I was concerned about it being either too loose or too tight (read how to know if you've got a good fit the discussion at the soloracer info link I mentioned above).
I noticed that Bell has the narrowest head circumference ranges for any manufacturer, so I went with a Bell helmet that was exactly the size of my head (it happens that surprisingly I take a small). The helmet fit exactly right.
Finally, I'll repeat back to you the advice I received: "Order your helmet well in advance of the time you need it to allow for the possibility of a return for a different size".
Hope you find what you need.