Pretty much anything you want. We've had very good results with Fluxlight SFPs for just about everything, including bidi.
Adtran, Cisco, and some others have a "profit protection flag" set by default on some of their gear which either shuts down the port or warns of plague, pestilence, volcanoes, and earthquakes should you use anything but manufacturer-branded optics.
In truth, all of the major manufacturers buy their optics from a company in China named Finisar. Finisar puts a sticker on them with the company's brand and programs a 256-byte EPROM with a vendor code and serial number. The company then marks up the price by at least an order of magnitude, sometimes much more. They all roll off of the same assembly line regardless of brand.
The switch firmware reads the EPROM and throws a flag if the vendor code doesn't match.
To turn off the profit protection flag:
Cisco:
service unsupported-transceiver
Adtran:
no alarm enable pluggable unknown
You can also reprogram the EPROM with programmers available on EBay if you do a lot of them.
The sales people will of course tell you that there is no support for non-branded optics, but you can have a dozen or so generic spares on hand for the price of one manufacturer's sticker.
