I never heard optcore either, but I always choose the optics from the third party manufacturer. They are cheap but high quality. Normally, I would buy optics from monoprice, C2G, and the mentioned FS.COM. They are cost-effective and fast shipping.

OM3 and OM4 are aqua or blue. OM5 is green.

The maximum transmission distance of glc-sx-mmd is 550m over OM2 MMF(Multimode). So using singlemode fiber would not work.

You may try FS.COM, I bought optical transceivers several times from there, and they work great until now. For the compatible BIDI GBIC, you can try this:Cisco GLC-BX40-DA-I Compatible 1000BASE-BX-D BiDi SFP 1490nm-TX/1310nm-RX 40km DOM Transceiver F...

Hi, I think you should upgrade fiber to OM3 or OM4 when the link distance exceeds 33m. The Cisco 10GBASE-SR Module supports a link length up to 33m on MMF(OM1).  Using 2000MHz*km MMF (OM3), up to 300m link lengths are possible. Using 4700MHz*km MMF (...