mongolian visa

i am planning on taking the trans-mongolian to beijing, in the process of
organizing visa`s. my question is:

1) how diffcult is to arrange a letter of invitation from mongolia?

2) what do i fill out underneath this heading in the mongolian visa
application?
21. Address & Phone of the travel agent



is this the angency from mongolia that sent the invitation?



3) does anyone have contact numbers/websites to arrange invitations?





thanks for your time,



richard from canada