I've taught myself a lot about slide carbs the last couple years ...learning some of their nuances and potential for throttle response and control. Finding a jet with a higher or lower number than the one you already have isn't exactly easy due to fitment issues, but jetting isn't the rocket science here. The holes are either bigger, or smaller. Jet needles on the other hand are more of a black art. Needles have fitment issues, like jets, but also taper issues, tip issues, diameter issues, clip position issues, and mid-point issues (aka: "L1"). Your average Mikuni carb can have dozens of different needle types made for it. And I've been selecting and purchasing particular needles for this one Nibbi PE24 carb I'm still tuning (which I've determined uses needles equivalent to the Keihin PWK28).
Still, needles are hard to research, and apparently even harder to find for sale. Try searching eBay for a certain carb's needle types and you'll generally only find a couple standard sizes by a few popular codes, ...or maybe just "needle fits VM22". That's been my experience anyway...
I found the JetsRus site to be organized and having the info one needs to navigate these matters. They have a good inventory. I've spent some money there. But alas now I'm arguing with them over a defective product they sent.
So yeah I'm getting really into needles, for Keihin PWK anyway, and as a matter of principle I have to shop for another vendor. Well last night I found Allens Performance. Their site is similarly well organized. It's also well stocked with the usual jets and other accessories, but also some pretty obscure needles even the other guy didn't have. Shipping costs seem really reasonable too. So stay tuned...
And that's all I've got for now. But chime in if you want to talk jets or needles!