Discs for Pioneer DVR-105

I got 10 free dics with the drive, Writek 4X -.

They burn great (my software reports 4.2X speed), no problems so far, however I have found you can only buy them in singles, and they are quite expensive.

I’m after some 4X discs that come in 50’s or maybe 100’s they WILL burn at 4X, good quailty, but good price.

I’m based in UK, any recommendations??