I’m not entirely clear as to what you want. This script takes a bunch of folder selections from the user and looks for a particular filename in each one until it either finds one or runs out of folders:
set fldrList to {}
set doneFlg to false
repeat until doneFlg
try
set srchFldr to (choose folder with prompt "Select folder to search:") as text
set end of fldrList to srchFldr
on error m number n
if n is -128 then
set doneFlg to true
else
error m number n
end if
end try
end repeat
set fileName to text returned of (display dialog "Enter the file name:" default answer "")
repeat with aFldr in fldrList
set srchFldr to (aFldr as text)
set fndFlg to false
try
alias (srchFldr & fileName)
set fndFlg to true
end try
if fndFlg then
display dialog "I found it!" & return & srchFldr
exit repeat
end if
end repeat
if not fndFlg then
display dialog "It wasn't in any of the folders"
end if