you can register at Hombu dojo and pay a monthly fee, see here for more details (click on Guide)
My association in the UK is not affiliated with the Aikikai, nor did I have an introductory letter. I never had a problem registering, although I do speak Japanese which helped
They do have a daily visitor fee, but you may find that registering is cheaper than paying the daily fee, depending upon how many days you intend to train there.
If you are already registered with the Aikikai, take your membership card (and yudansha book if you have one) with you, it will probably smooth the path.