Shimano includes a pair of downtube shifter boss mounted cable housing stops with every aftermarket pair of STI brifters so most bike shops have a box of leftovers. They should be happy to sell you a pair at low cost or even give them away if you are a well known customer.

If you can't get a set locally, here is an internet source: http://www.loosescrews.com/index.cgi

#1 stops have a flat back and will fit ony size downtube.
#3 stops have a curved back and are specific to 1-1/8" diameter downtubes, which you have.

As Homebrew101 noted, your frame' rear dropouts are most likely spaced 120 mm and a modern 8/9/10-speed cassette hub will be 130 mm. It will probably be necessary to have the dropouts "cold set" (i.e. permanantly bent) to accept the wider hub without a fight. Your LBS should be able to do this and realign the dropouts afterward. Also, have them check and adjust the derailleur hanger alignment. Modern index shifting, particularly 9 or 10-speed, is far less tolerant of indifferent alignment than friction shifting was.

It's a bit late to say so but this conversion is going to cost a lot more than paper boy money, even after you bought the brifters.