By shims, do you mean those little clips that hold the pads in? Or the backing plates; I think those come with new pads don't they?
You can get them as a set from Nissan, but you may need to give them the part number.
Try part: 41080-54C26 Hardware Kit-Front Disc Brake Pad - That should contain both the clips and the backing plates for one caliper
Try: 44060-75C90 Pad Kit-Disc Brake for the rears.
You could also try the Sunny GTi (i.e. SR20DE engined) parts because I think it's the same caliper at the front... I don't know about the back; that's a unique caliper because none of the other related models have a driven rear axle. - However, 44060-0N690 is a superseding part that fits the N15 Almera, so that's probably worth trying.