Up by Peterhead is the main source. Can't say what it's like as I've not been. I stopped by one day but was just quickly searching the beach and didn't find much. Later found out that the seam is up by the fish farm at Sandfordhill above Stirling village. That site is protected but flint can apparently be found in the surrounding area as well.
I've been using pitchstone from the beach at Corrigills, just south of Brodick on Arran. There's a regular little cliff of the stuff runs along the coast for a couple of hundred metres then off out into the sea, and most of the shore is black glass boulders from from fist size to about 3m high.
The next time I'm over I'm going to try to get to another seam, partway down the cliff at Drumadoon on the west coast of the island. I'm hoping it's the source of another type of pitchstone, light green in colour and by the looks of things it fractures nicely.
There's Jasper in the Campsies, not a great deal of it by the sounds of it. I've got one circle on a map up near Birkenburn Reserviour and another in the gully below the quarry in Kilsyth. Just circles on maps though, haven't checked them out yet.
On the whole, flint can usualy be found where chalk is, but I've never spotted much chalk in Scotland(Don't have a decent Geology map of up here).
Here's the couple of links that got me going.
http://www.soton.ac.uk/~imw/Geology-Britain.htm
http://www.oum.ox.ac.uk/onlinedb/maccull/machome1.htm