yes you can keep them together. we have not in the past, mostly because our sheep are trained to electric fence, and our goats were not. there are the usual issues... if you have both a buck and a ram in the same field, you may have issues, or you may not. some groups get on fine, other may take a while or may not ever. my first pair of goats were pretty sure that goats are what sheep have for lunch... couldn't keep them in the same pen, no matter the 6 ft fence, the goats were 1-leap-over-the-fence, eyes as big as saucers and running for the hills. my current herd & flock share a fence line so they're not naturally terrified. where we have just moved to, they'll be sharing a pasture, once they arrive later this month.
there are some feeding situtations to be addressed. goat minerals, supplement blocks and pellet ration are too high in copper for sheep, possibly lethal over time, so depending on your type of feeding arrangement they may have to be separated for feeding. they will both graze, but as noted above, sheep graze then browse, and goats browse and then graze. my goats generally prefer leaves and branches to good grass hay, although alfafa trumps both. sheep and goats can both strip the bark from your trees. there are some poisonous plants that sheep or goats are more reactive to, but in general they can eat the same things.
how difficult to find... that depends on where you are. there's one sheep dairy in CA and they don't always sell ewe lambs... just depends on what they're doing with the size of their flock. some of the northern states have sheep dairies, and I know there's at least one in maine. there is an increase in interest right now in dairy sheep, so availability may be changing.
what area are you in?
as far as "good" dairy sheep, I'd definitely be buying from a production dairy... they track production, know what bloodlines are producing, and breed for both health and milk production. they won't be cheap, but you'll get good stock. you might also ask at what age they start to rotate out production ewes. you may be able to buy ewes that are past their peak production years, but still have good or fair production and can make you lots of lovely dairy cross lambs.
we've found the dairy line ewes typically twin and triplet fairly often. with triplets, we watch to see if one of the lambs is less competitive, and if there's one that's not keeping up, we bottle supplement that lamb, but we leave all three with the ewe. dairly ewes will make enough for three lambs, but sometimes two of them are hogging the milk
the dairy ewes are good moms, can count to three (some of our other sheep have trouble counting two lambs) and lamb easily. you will need to provide some high-calorie and high-calcium feed while they're in milk, they do lose condition more than our other sheep while lactating, probably because they're producing so much more milk.
did I get all your questions?