I got a NI Plate for £25.
I thought it was too good to be true, but thought I am gambling £25, not £200+
To be sure I called them and asked why they were priced so low. The explanation was they get them in bulk from NI, and sell them just to cover their costs and it attracts people to their web site.
I ordered one, and a few days later received a call from them. They apologised and said their was a typo, and the last digit of the plate I had chosen was a '5' and not a '6', did I still want to proceed? I said yes.
The process took a little longer than I had hoped and after ten days or so I contacted them. From that point onwards I got regular emails giving me updates, and too be fair the delay did appear to be at the DVLA.
If you Google the company they have had CCJs against them, and a bit of a mixed reputation. I went with them as the amount I was gambling was small. The paperwork came through after about two to three weeks from the DVLA , and all was fine.
The company I used was was www.R4V.co.uk and they appear to currently have NI registrations from £29
I am not giving them an endorsement, but it worked for me. I would suggest you call them first and make sure they have what you need, and talk about the turnaround time.
Good luck.