You need an ECE 22.06, or better yet, SNELL rated full face motorcycle helmet. DOT is legally acceptable in the US but get better protection than the bare minimum.

Even 25mph is plenty fast for life altering (or ending) injuries, you can get messed up going even slower than that and the faster you go the amount of energy involved in an impact goes up way more than your speed increases.

Get yourself some motorcycle gloves and consider a jacket/pants/boots too especially if you're going to be riding frequently.

Vendorwise, revzilla or motosport should be fine.