If you fill in your details on the DVLA Sale-of-Marks site HERE they should email you when 05 plates go on sale. Generally, they start to sell plates a couple of months before the registration is live - you have to be quick to snap up the popular combinations.
Avoid placed like NewReg etc - they overcharge. DVLA is the only place where new plates can be purchased. Buying from elsewhere means someone is taking a cut. When I bought my plate a few years ago (old Prefix style) it was £250 fully inclusive from DVLA. However, places like NewReg quoted £400+
The only time other sellers are handy is when they already own the plate you're after, so that's the only place they are available!!
EDIT: also worth noting - when you buy a plate, the retention certificate is valid for 12 months. You will have to renew the retention every year or you will loose the plate. I think there is a small-ish fee for this, and there is also a tight window of when you can do it. To revent this, you could always assign the plate to a car, but remember, you don't actually own the plate - it is on the car. If you sell the car you must transfer the plate off before the ownership of the car is transferred. If your car is stolen, you have to sort it out with DVLA before your insurance pay out. Finally (and this happened to a mate), if you have a personal plate on an old car, you must keep it in good working order. To transfer a plate from a vehicle, it MUST be taxed and have a valid MOT. My mate had a plate on a car, but it got into a really bad state of repair. No MOT, no way he could transfer it to another car, so he lost it
Cheers
Phil