The ones I used to get from mosti said vinifera raisins on the bag. They were very large berries double the size of your regular sunmaids and they were red with thick skins. They had white ones too but I never bought any.
ABC Cork also sells bags of south african vinifera winemaking raisins , they are a canadian distributer so any of their retailers can get them. They have a presence in the US too.
thompson seedless are as you say not the best ,
If you can't find winemaking raisins , then I'd reccomend the zante currants over thompsons , Zante Currants are not actualy a currant but a vinifera grape call black corinth. you can sometimes find these grapes fresh in the produce section , sometimes called champagne grapes , you can reccognise them because the berrys are black and very tiny in tight clusters.
I would want to minimise any stems , others may want them . to each their own, if the stems were nice ripe and brown when harvested it could potentialy give some nice character .
My prefered addition is not raisins as they by nature are oxidised , as is any dried fruit . But they are still a worthwhile tweak.
If in season you buy a lug (36 lbs) of red wine grapes , you can hand stemm them fairly easily and add them to a 5 lb ziplock bags crush them and put them in the freezer. then when you make a kit add 1 or 2 of the five pound bags to the must .
let them thaw over night in the must and pitch the yeast the next day.
you get nice aromatics and the concentrate flavours are minimised . this is my favorite red kit tweak , it realy brings out the flavours adds some tannins and gives the wine a much more commercial wine charateristic. one or two bags of grapes won't throw the kit all out of balance in my expireince . don't worry about matching the variety exactly with the kit , a small amount like this won't mask the kit. a medium wieght grape like merlot is a good choice, but any ripe red wine grape will do. if useing cab sauv make sure its ripe or it can taste a bit green.
here's a thread on adding grapes I did on the other forum
its a port kit but the grape part is the same for any kit.