General advice - Best thing is to go once with no intention of buying, and watch the trade. Then go and choose some you like the following week, and bid on them.
You're a little early for store lambs yet. I can't see that Lanark are selling stores yet; at least there are no reports for store lamb sales this year yet. It looks as though they start selling store lambs on August 12th.
Longtown's opening store lamb sale is a week on Tuesday, 29th July. According to their website there will be some store lambs to sell this Tuesday, 22nd July, but I think it'll not be many yet.
Dumfries' opening store lamb sale is 22nd August.
It looks like Stirling are selling store sheep on Wednesdays, so may be worth popping along there this Weds. But as there were no store lambs sold Weds just gone, I think I'd be inclined to give them a call and check that they are expecting some to come through