The snap tape is sold here
. I found out through trial and error several years ago that RV snaps are not compatible with the generic snaps available at fabric stores. It is, however, available (although most rv dealers sell them mounted on plastic, which is not useful for curtains).
You will need to sew it in to the back of the curtains. I ended up cutting them up, singing to seal the cut ends and then hand sewing them into my curtain lining, since I had already sewn in pinch pleats. How far apart you space the snaps is entirely discretionary--just do it evenly.
The new curtain railing from Vintage Trailer supply appears to fit the old sliders--but I haven't measured ours yet to be sure. If they fit, I'm definitely going to buy replacements for the old paint encrusted ones we have now.