The largest base would be this site itself, I think.
Archived beginners' contests - http://logicmastersindia.com/lmitests/downloads.asp?testType=10
Archived Monthly contests (higher difficulty, more variety, closer to the WSC format) - http://logicmastersindia.com/lmitests/downloads.asp?puzzleTypes=S

As you can see on the above link, there were 7 Sudoku Mahabharat rounds last year and that has been announced again this year, with dates to be released soon, so just keep tabs on the forum and participate in this year's 8 rounds, and also look into those archives and time yourself in an entire contest for practice. Solving an entire contest simulates the actual competition scenario way better than solving single Sudokus at random.

There is also the World Sudoku Grand Prix which unfortunately just got over this year but will start up again next year most likely in January. This is the site - http://gp.worldpuzzle.org/

If you are interested in just leisure time solving there are many great blogs by world class Sudoku authors which can be reached here - http://logicmastersindia.com/home/?g=links

You can also check GM Puzzles - http://www.gmpuzzles.com/blog/ It is largely a puzzle site but there are a good number of Sudokus too.

At the start I mostly just participated in LMI contests and downloaded older contests to time myself. I do puzzles too (and am more of a puzzle fan than Sudoku) but I don't think that has a huge correlation to my speed other than increasing my adaptability to newer variations.

Hi, This is R.chandrashekar from Bangalore i had particpated in TOI sudoku championship2015 which was held on 26th July @St.Patricks convention hall.,I was shocked to see my scores and time,I clearly remember that i had completed all the sudokus C1 to C6 and C7 correctly,but your score card shows that points as 0 for C1 and C2 but iam damn sure that i have completed all sudokus correctly(i doublecheckedit)., something has gone wrong while evaluating.I want my papers to be re-evaluated .And one more thing is that the time record shows as 60minutes in result, where as i had completed all the sudokus in 49min and 15secs i had noted down the timings as i submitted the papers to your representative and i saw him writing the timings also .Iam requesting you to re-evaluate my paper and let me the status of the same.

With best regards,
R.Chandrashekar.
Mob:-9740396237.
Bangalore.

We set submission time to 60 for all players who aren't eligible for a bonus. If you are eligible the time bonus note will also be considered. Your paper will be rechecked on 10th/11th.

Thanks Prasanna. It would help me to improve a lot.

Complete Results from Finals - http://logicmastersindia.com/2015/TSC/

We have re-evaluated your sheets, highlighted the mistakes, and sent you a mail of the same.

Thank you very much ,will correct my errors and come back next year.
regards,.Chandrashekar.

You can correct the errors by participating regularly on contests here through the year

Hi my name is varalaxmi I am from Hyderabad. I want to know is their competitions going to held in Hyderabad and also suggest me how to participate in competition regularly held.