Hi there. If oxygen is not toxic to your bacteria and they are not heat-sensitive, you can add another agar layer after spreading/striking the plate. Or you may use "pour plate" to culture your bacteria.
The old-fashioned way was to incubate plates in a sealed glass jar and a lit candle – the candle will burn briefly after the lid is put in place, removing oxygen. This works well if you are careful and so long as the CO2 produced is not a concern (it could effect the pH of solutions / plates).
Culture remains the gold standard for the identification and speciation of anaerobic bacteria .but a deluge of chemicals, reagents, media and equipment are commercially available for anaerobic study