Bprojects sells both:
* GX head gasket 3780 Yen (about $38 USD)
* Nismo head gasket 12960 Yen (about $129 USD)
http://www.datsun1200.asia/category/select/cid/357

Sometimes you can find them on eBay, if a guy has one to sell, not every day. Kameira is the only other place I know of with regular stock, and they are very expensive. Ten years ago Nissan sold them but no longer

Does the bore size matter. Should I be getting the 79mm gasket?

Yes it matters, but not enough to tell. Most head gaskets are bigger than the original Nissan items, so they will fit a variety of engines (like A12 & A14 both, whereas original A12 Nissan gasket was different bore size from A14 Nissan gasket). So most of us use a commonly available gasket

For standard A15 pistons, you only need a 76 mm gasket, but can use a 78.5 or a 79

You could custom order a copper gasket from Cometic was around $100 last I checked but it's been a few years. Just depends on where you want to spend the money

I just had a look at the link you provided. They seem to be mainly 79mm. Once I check all the cc's and get all my figures I'll decide on the gasket. Now checking the piston to deck height is a straight edge and feeler qauges sufficient enough or should I got a deck height gauge? My Pistons are def below deck height. Just by eye I would say close to 1mm but I guess I won't know until I measure properly.

Feeler gauges are accurate enough. Alternatively you can CC the piston in the bore to measure dish at the same time

With pistons down in the bore, you won't be able to get adequate quench. No worries, it'll still run well, although not with optimum power