Remember that Lexar made in the past 2x128MB memorysitcks that worked on old Cliès (total of 256MB on two separate 128MB banks).
Ha ha, that is as rare as an albino pigeon.
Furthermore even 1gb memorysticks are getting rare. Your best bet is to find a memorystick adaptor to hold pro duo memorysticks that were given with PSP, they come in 8mb, 16mb and 32mb sizes and can be bought for under $5.
Th-55 U, T/X, TG-50, Handspring Deluxe (3) and PSP user.