1N5303 is available from several suppliers:
Central Semiconductor, Digitron, Microsemi...
I'm not sure if it's easy to buy from them and it's worth it.They are manufacturers, I don't think they even deal with individual customers. Also, afaik, both known to produce expensive absolete parts that are not produced by other companies. They don't even have prices on their website.
Mouser lists 1N5303 for ~2.5euro (I guess VAT is not included) and as not normally stocked. Compare to much cheaper fets.
So, they are in production, but hard to buy anyway. I've also seen them on ebay, but price was what I'd call unreasonable.