I searched on the # Mitch provided earlier and it appears they're available from several sources such as Western Depot which claims to have over 25 in stock at $8 for a 2-pack. I don't use Superstreets myself so don't know if that's a "good" price but seems reasonable to me.
If you choose to proceed using the 153IR and need specific wiring instructions, be advised that there are (at least) 2 versions of the 153IR that have slightly different hook-ups. So when I suggest using the "NC" output, this may not be enough info. We can drill down to that level if need be...