You can deal with DVLA to obtain a copy V5 in your name. This is a reasonably lengthy process (last time I did it, it took about 7-8 weeks) and you will need to provide clear proof of purchase / ownership to become the registered keeper.
As an aside, garages can register a car (and while I don't know the ins and outs) and class them as pre-registered so they can sell it cheaper than the equivalent 'New' car. There are some tax implications on their part and therefore they aren't supposed to sell them with 3 months of registration. This could be the underlying reason behind your delayed (maybe not missing?) V5.
Before contacting DVLA, I would be inclined to go back to the supplying dealer and try to get someone who knows what they're talking about... the V5 may just be on its way to you just now....?