I have boxes of TDK DVD-RW @ £3.90 a box of 10 from Dabs.
Dual layer DVDs (DVD9) are DVD+R DL, although drives capable of writing the DVD+R DL format may also be able to write the DVD-R DL format if it is released.
Be careful of what you buy. Infiniti DL do not work with LaCie(NEC 3500), Verbatim do, all down to dye type.
RW DL, if only !
Panther 10.3.8 with Toast & Jam.