It's probably in the glove box of your car!
The DVLA doesn't deal with MOTs. It's VOSA that administers MOT testing and determines the maximum fees that garages can charge for providing MOT facilities. The maximum fee for a replacement certificate is £10. As far as I can see, you can only get a replacement certificate from the garage, not from VOSA.
As you suggest, you don't need an MOT certificate to tax your vehicle online (or by phone). If you were selling the car you should be able to persuade a potential buyer to accept the evidence of an online check on the vehicle's MOT status. You only need the registration number and V5 document number for that:
http://www.motinfo.gov.uk/
So you probably won't need to possess a certificate unless you're given a 'producer' by the police. That's extremely unlikely, since most police officers would now probably run a check on the tax, MOT and insurance status of the vehicle at the roadside, rather than waste everyone's time by forcing you to produce paper documents.
Chris