Furuno, RayMarine, Garmin, JRC, Sitex, and others make small boat radars. All modern radars have perimeter alarms.
Compared to AIS, radars for small boats are expensive, complex to install, and power hungry. AIS gives pretty much the same info (alerts to commercial shipping over 65 feet LOA in the vicinity, distance off, course, and speed.)
12 boats carried radar in '08 SHTP. On my boat (27',) I used AIS 100% of the time, and radar less than 10%. And that 10% was to alert for approaching squalls, not shipping.