I just looked at Railway Prototype Cyclopedia's website and all the recent issues are sold out. I highly recommend you contact your supplier and see what they have left. I just received Volume 34 and it is full of corrections and additional information from the previous issues.
Volume 1 Volume 2 (sold out) Volume 3 (sold out) Volume 4 (sold out) Volume 5 (sold out)
Volume 21 (sold out) Volume 22 (sold out) Volume 23 (sold out) Volume 24 (sold out) Volume 25 (sold out)
Volume 26 (sold out) Volume 27 (sold out) Volume 28 (sold out) Volume 29 (sold out) Volume 30 (sold out)