Yes, Tri-X is available in bulk but now that the Arista stuff (well I guess you aren't in the US so shipping might have been prohibitive anyway) is no longer available it's expensive, though still cheaper than individual rolls. I only have US prices but it's $69.95 at B&H. The usual figure quoted is 18 rolls of 36 exposure (I've never kept track) so that would work out $3.88 a roll. 36 exposure rolls are $4.49 from the same vendor. Not worth the trouble, to me anyway, IF your primary motivation is to save money. I usually load shorter rolls and being able to do that is nice.
As for the other question I guess it depends on how slow it needs to be. FP4+ is a great film and with Plus-X gone probably the last "mostly conventional looking" medium speed film. There is a shortage of slower films. I like Pan F+ but it's sure nothing like a slow Tri-X! There are also the Foma films which are usually best about a stop under box speed so you could get the 100 and shoot at 50. I've never shot the 100 but shot some of the 400. It's a decent film, grainy, but that may be fine and wouldn't be as much of an issue in 100. In fact I wouldn't be surprised if the grain of the 100 is similar to Tri-X (or, more likely, larger.)