"Showbiz Ballyhoo." Video Late Show/I.V.E. (No list price.) In addition to those wonderful Hollywood movies of the '30s and '40s, there's a wealth of campy, corny and just plain nostalgic frills from the period--previews, promos, home movies, screen tests, bloopers and other outtakes. One of the great things about home video is that it encourages producers to find and assemble such long-unseen material, and this 76-minute tape is a good example. It's an uneven but largely fascinating mishmash--including Vivien Leigh's and Joan Fontaine's screen tests for "Rebecca" (Joan got the part), goofed-up lines by Ronald Reagan and others, and glimpses of a delightful pool-side 16th-birthday party for Judy Garland. Should sell at stores for about $15, according to I.V.E. Information: (818) 888-3040.