Young man in shopp dress from Tran, Bulgaria. 1921 (sourced from Wikimedia Commons)
If you are unable to source any sirene (сирене...which simply translates as cheese, although I am told it's specifically white cheese), follow the advice of my lovely Bulgarian friend, Velin Bachev (do check out his work - he is a brilliant sculptor and musician!), who confirms that feta would be an acceptable substitute. He also says it's okay to crumble the feta/sirene instead of grating it... have you ever tried to grate feta? Just don't - it's not pretty! Make sure you rinse and dry the cheese really well (with kitchen roll), which makes it easier to crumble. If using feta, don't salt the salad because there's enough salt in the cheese. Unless unlike me, you are fond of overly salty food! I probably didn't use as much sirene as some of the Bulgarians do... but they probably don't need to watch what they eat like I do!