BBT Charting and OPKS and VITEX

I wanted to start charting my bbt and taking OPKs but heres the problem i work at night and i dont know if my bbt would be accurate and i dont know where to buy the thermometer at 🤔 And Secondly the Opks my last period was in the end of APRIL...I havent had once since then (im irregular) and i wouldnt know when to take OPKS because of that....