Whilst I advocate buying from a local Music Store via an Online purchase if this is not viable then there is always Amazon.
A Yamaha PSR e583 is currently £419.00 with free delivery & free no cost return.
Do note that in addition to the keyboard you will require a keyboard stand and stool. I suggest that you do buy a keyboard stand by a reputable make, such as On-Stage, K&M, Roland etc.
There are four basic types of Keyboard stand, X brace, Z stand, Table & Column.
X brace tend to be the cheapest, if you do buy one of these do get a heavy duty version and always use a safety strap between the top two arms. A double X brace will be more stable that a single.
Z stands give more legroom when seated and then tend to be more stable.
Table, as the name suggests, there are Pros and Cons but they do look better in a home environment.
Column, look fantastic but cheap ones are not stable in that as you play the keyboard can move.
This is what I use, a K&M 18820 which costs £132, but it does support my Korg Pa arranger.

An On-Stage heavy duty X brace will be about £70.
