Burner cans for the IIIE/O are contained in a few conversion kits for Mirage III derivatives like the Mirage 5, Nesher, Finger, Cheetah, Pantera etc. Usually these are for the Heller Mirage IIIC kit or the Esci Mirage IIIE so not sure how they will fit the Eduard/Hobby Boss IIIC. You can try Spinners for their Cheetah conversion kit (see Linden Hill's website - they may have some left as John has stopped producing and they are sold out pretty much everywhere else) or Eagle Designs (Eagle Designs), who may sell you a burner can and back end alone. I don't know if High Planes do any burner cans in 1/48 but you could try them too - they at least do some bang seats for the IIIO.
In addition to the length, I think you may need to adjust the intakes too. Not sure about this but I thought the intakes on IIIO were larger in the same way as on the two seat IIIB.