Ten heads in a row has a probability of 1/1024
Ten tails in a row has a probability of 1/1024
Ten heads OR tails in a row therefore has a probability of 1/512 (as you said)
If you do only ten flips, then this is your chance of getting that run
If you do eleven flips, then you have two chances to get the run - the first ten flips, or the last ten flips. In this case the chance of getting a run is 2/512
If you do 12 flips then you get three chances - the first ten flips, the middle ten flips or the last ten flips. Chance this time is 3/512
Working up from this, if you do 100 flips you get a probability of 91/512
I'll let you do the last step yourself
Edit: This is actually all wrong the more I think about it, as the two chances to get a run of 10 with 11 flips, for example, have a large area of overlap in the middle and so aren't independent. In that case you actually need to work out the odds of getting a run of 9 and then the odds of getting one more head or tail in two more flips, and the odds of that work out at 1/341 not 1/256 as I said above.
However.... if you have 10 independent tries at getting a run of 10 then the odds of getting one then work out at 1/51.7. For a run of 100 though the odds must be better than this as there can be an overlap between tries (getting 3 in a row at the end of one run, then 7 in a row at the start of the next for example).
So I'll say the odds must be somewhere between 1/5.6 and 1/51.7 then