One way is to define a bunch of random seed locations within the domain like this:
N = 75; % number of seed locations
xstart = max(x)*rand(N,1);
ystart = max(y)*rand(N,1);
For example, try this:
x = 1:200;
y = 1:200;
[X,Y] = meshgrid(x,y);
vx = 30 + 30*cos((Y-100)*pi/100);
vy = 5*cos(X/10).*cos((Y-100)*pi/100);
figure
pcolor(X,Y,hypot(vx,vy))
shading interp
N = 75;
xstart = max(x)*rand(N,1);
ystart = max(y)*rand(N,1);
h=streamline(X,Y,vx,vy,xstart,ystart);
set(h,'color','red')
