Hi.
If the Metformin in the cupboard is still in use by date, then they may be OK to use. However, bear in mind that if you were put on slow release Met, was that for a reason ? Stomach upsets etc ? Also I hope that you remember the doseage you were on.
If none of these things apply, you can always get 'emergency supplies' from a Pharmacist. There is paperwork involved though. A couple of days without isn't going to be too much hassle although you will need to be careful with what you eat. Not too many carbs as these will elevate your Bg levels.
Have a Happy, Healthy and careful New Year.