Retailers apparently have to order stocks of film from Fuji a long way ahead, of course it is difficult to anticipate demand accurately, so some emulsions are in short supply. I have just checked Speed Graphic's website, they show most Fuji slide films as being in stock, in 35mm anyway.
If you are thinking of Kodak, the 400 iso variants, are in my opinion vastly inferior to their Fuji equivalents, but the 100 iso stocks seem to be fine, although they have different characteristics to Fuji. I tend to use Fuji, but do use Ektachrome EBX on occasions, the colour rendition is saturated, but rather more neutral than Fuji Velvia 100 or 100f. It is a bit like a slightly more saturated Provia (to my eyes anyway), but may be slightly grainier.