3 week old lambs would need 3 feeds per day roughly, 4 week old lambs 2 feeds, 5 week old 2 feeds of half the amount they have at 4 weeks, and 6 week old lambs just 1 half bottle in the evening with the aim of weaning at the end of 6 weeks latest. Or that's how we've always done it and it's worked for us. Creep topped up twice a day and then put out to grass.
Orphans aren't a cheap option though, each lamb will go through approx £25 of milk powder, say 1 1/2 bags of creep at £8.80 a bag, they need worming, heptavac etc. I worked out last year each lamb we reared cost us £65 including the £25 kill and cut, and we had a good year where nothing went too wrong.
Given that store prices are low at the moment as most people don't have much grass to finish them on, I would probably buy some more stores this year and rear a few orphans next year.
If money is short I would probably not have orphans this year, if you have too much land I would section it off and cut for hay. I think you would be pushed to find orphans for sale much older than 3 weeks as others have said, most of the hard work is done and they become a lot less labour intensive.