Well a reliable source is really difficult to point. "Reputed breeders" may be one reference for you. I personally have seen everyone has "sales" at the back of their mind. Specifically you yourself have to choose the best pup, which is really difficult for first-time dog owners. Reputed breeders generally does not sell a bad quality pup but they cost a little extra(simply for their name)...what you will get at 12K, they will may be charging you 15K....if you are ready to pay this extra then go for them. Pups from domestics will cost you less but you will have to recognize the quality. Normal Pet shops also sell good quality pups but very dubious...again you will have to recognize the quality which is really difficult for first-time dog owners. Best solution might be, go along with someone who had has a LAB and not just any dog....lest you find no one as such then you may go with a person who has fair bit of idea about dogs. I say this for definite reasons.
I have personally seen Lab Pups @4k and 17k...both were lab indeed but there was uttered difference...!!!
LAB can cost you as low as 8k and as high as 35k or even 1 Lac(Original British-imported best quality show pups from champ-line with KCGB chip)!!! Anything between 8-12K is fair enough for you,I guess.
All the best!!
[Prices as is in Kolkata. I just shared my first hand experience.]