Wayne, if you're looking for one, Klingspor's has a top bearing bit in 1/4" shank (EDIT that is a flush trim bit with a bearing END EDIT). as that's what I bought and used (as seen in the pics in my earlier "Exact Width Dado jig" post; I can provide whatever info from the package if you need it). THey may also have a bottom bearing bit too, as that's more common I think, but I was not looking for that so I can't comment. Others vendors likely also carry such things.(Just wished I had found a bearing guided 1/4" shaft with a bearing so I could have used trim router ie Colt.)
Wayne
Google "Making your own T tracks." Lots of good info in you tube videoOrdered a cheap set from Amazon. Should get the immediate jobs done. One for 1/4 bolts, one a little larger.