There are some kits on Amazon which work well, and have different drug categories so it will test for methadone and other drugs etc. The lost important thing to consider is OD – do you have Naloxone? And or consider giving some TIPS on keeping safe, avoiding using alone, In large amounts, testing strength and tolerance levels, as well as practising good hygiene. It’s not uncommon for people (I’m assuming Heroin) to relapse, especially if they miss a dose they would need a substitute.
I’m sorry you’re going through this, I’m sure it is really worrying for you, however the safety plan is necessary.
Good luck with it all.