Sensitizing dyes in general run about $100 USD / gram. But, since you only need a small quantity such as 10 - 100 mg / 109 g of Silver, this is quite a bit of dye to work with. For red sensitivity you need a dye that sensitizes in the region of 650 - 700 nm. If you go longer, then it is an IR dye.

There are several dyes that I have posted here that can be bought at Sands. The red dye is a "J" aggregate former though and is only a red sensitizer on a Bromo-Iodide emulsion. It forms 2 layers that turn a green sensitizer into a red sensitizer. It is, in fact, the dye used in Kodachrome.