In our area most beginning umpires of 11-12 year old baseball use a league supplied external (balloon or raft) protector. It is not a bad way to start since you are not likely to get hurt when the catcher misses the pitch so you won't develop bad habits. Many umpires around here use the Diamond ribbed protectors like the ones at this online source
http://www.btlofficialsgear.com. They are probably the least expensive new chest protectors. I have also seen them at some sporting goods stores. I use an Itech Davis protector, similar to the Wilson Davis protector at btl and
http://www.gerrydavis.com. It has done fine for me through high school ball.