This list is as of Spring 2012, and the orignal can be found here:
http://gepikbibimbap.wordpress.com/gepik-coordis/
 
Ary Kim (Head Coordinator) - ALL HIGH SCHOOLS:
031-249-0234, arykim@goe.go.kr
 
Victoria Oh (Goyang District) - Goyang, Gimpo, Uijeongbu, Dongducheon-Yangju, Paju, Yeongcheon, Pocheon, Gapyeong, Bucheon ELEMENTARY AND MIDDLE SCHOOLS:
031-900-2858, victoria@goe.go.kr
 
Gina Kim (Anyang-Gwacheon District) - Anyang-Gwacheon, Suwon, Ansan, Pyeongtaek, Hwaseong-Osan, Siheung, Gwamyeong, Gunpo-Uiwang ELEMENTARY AND MIDDLE SCHOOLS: 031-380-7061, gina@goe.go.kr
 
JK Ko (Gwangju-Hanam District) - Gwangju-Hanam, Seongnam, Yeoju, Yangpyeong, Icheon, Yongin, Anseong, Guri-Namyangju ELEMENTARY AND MIDDLE SCHOOLS: 031-760-4062, mintjean@goe.go.kr

You might be thinking of a "Certificate of Employment".  It is a just a basic "just the facts" form that tells when, where, and other facts about your employment.  There is no set official standard.

My advice is to get one from the public school.  Alone it might suffice.  If possible I would recommend getting the letter translated and notarized by a Korean notary.   Maybe even apostiled by the Korean government.  That might be overkill.  What is the standard of proof being asked for in Taiwan?

You should be able to get one by contacting your former school. It's not uncommon for Koreans to ask for such a letter or document to confirm when they worked at a certain company. The law says that former employers are obligated to provide proof of past employment.

Just to be aware, any ex-employer has to provided the certificate of employment, after three years of employee retiring from workplace.

Will post/edit more later with an opinion on it.  Later.

You can ask your previous employer for either a 경력증명서 or 재직증명서. It shows the dates of employment as well as the employer and your position.  They will immediately know what it is. Sometimes you have to pay just a little bit of money for it. If you've worked in a private school, you can ask your employer to translate it to English and stamp and sign that. It should be on the school's official letter head. If you've worked for public school, it is all in Korean and they can't translate it for you in the office but maybe you could ask your co-teacher to translate it to English and have the translated copy stamped as well. Most people need this when transferring public schools to receive the correct pay grade so if any of you are transferring, be sure to get this before you leave.  I had to go around town and collect three to guarantee my pay grade this year.

Victoria Oh (Goyang District) - Goyang, Gimpo, Uijeongbu, Dongducheon-Yangju, Paju, Yeongcheon, Pocheon, Gapyeong, Bucheon ELEMENTARY AND MIDDLE SCHOOLS:
031-900-2858, victoria@goe.go.kr
 

Victoria is no longer our coordinator. Currently it is Ellen Kang and her email address is ekang7979@gmail.com.

Well what I did was obtain a certificate of employment from the first school where I taught in Yongin, and I sent that to them. ALso make sure that the certificate is stamped with that red symbol.

As Teacherwalters mentioned, it is important to have the schools stamp on the form or it's useless. Each school has their own stamp and it's considered the schools signature. Without it, you can't prove that it is a legitimate form. I will attach the form, but since you will need the stamp, you should ask your school for it directly.

I forgot to mention, this is what the GEPIK schools use, but it will most likely be useful for most of you.

Thanks! I should have no problem getting the stamp from the school.

Contact Samuel Wilson if you're with SMOE and he'll set you up with a letter.

Would they accept a copy of your old contract on top of the letters of recommendation and visa stamp? Maybe you could show a pay stub.