Before you buy your first sword it is wise to speak to a dojo senior about the weight, length and source.
It is really important that you do not buy a cheap wall hanger (for display only) sword, these swords often do not have a full-length tang and are a danger to you and everybody around you if used to train.
When buying your first sword you may wish to buy a new inexpensive model or even a second-hand sword until you have developed the skills required to minimise damage to the sword and saya, again take advice from a dojo senior, get them to check out any second-hand sword for safety, condition, and suitability. If you are going to buy a new sword, I would recommend going down to Nine Circles (you must book in advance and bring the required documentation to purchase a sword) where you can try a few swords and buy the one that feels the most comfortable. The staff at Nine Circles have supplied Ren Shin Kan Iaido with swords and clothing for many years and their advice can be trusted.
If you wish to buy a sword from Japan (Tozando etc.) be aware that you will have to show the correct documentation for HM Customs to import the sword and may have to pay import duty, this with the shipping can take some time.