I own German Shepherds and a Lab and have always had big dogs so I can't advise on that BUT I can say this:
As a rescuer I would urge you to adopt a pup or dog from rescue - national, local, breed or all-breed - as there are already more dogs in the UK than there are homes and thus healthy young dogs are killed in their hundreds each week in pounds and vet surgeries up and down the country. I'm happy to recommend reputable rescues and tell you what to look for and what to avoid in one if so.
If you are not willing to do this, FGS - or rather for yours, the dogs' and the future of canine welfare - please go to a reputable breeder whom you have found through the BREED club, not through the Kennel Club.
Please be aware that a huge number of Kennel Club reg dogs are puppy farmed - even reputable breeders reckon it to be about 90%. KC registration or accreditation is no guarantee of a reputable breeder or a healthy pup. At best these people are largely Backyard Breeders - those who breed for profit or because their dog is cute with no regard for improving or even maintaining breed lines and no bloody idea what they're doing.
Do all your research, ensure that your pup and his parents have had all relevant health checks and insist on seeing ORIGINALS of all paperwork to prove this. Avoid anyone who breeds more than one breed of dog or who has more than one litter. A good breeder will have a waiting list AND a back-up waiting list for their pups BEFORE the bitch is mated and will NEVER, EVER advertise their dogs in the papers/online - they don't need to.
Be prepared to be asked lots of questions by the breeder - the less they ask the less they give a monkeys where their pups are going to live and with whom, which speaks volumes for them. Check their license with their local council.
Bluntly, and very sadly, approach this with more caution than you would finding a childminder for your PFB as the number of puppy farmers and backyard breeders out there is huge.
Oh.... and forget this daft Lhasa Apso idea.... German Shepherds and Labrador crosses are much nicer! :o