Well... you don't have to look too far to find matches with phosphorous. I have a few boxes of strike-anywhere matches I got from Canadian Tire just a few years ago (3 or 4 years ago?). They are the 'Red Bird' brand or something like that... they're for lighting barbecues and fireplaces, so they're extra long and extra thick... with a generous amount of phosphorous on the tips!
As for applying it inside the case... here's what I would do: borrow a small wooden herbs mortar from the kitchen and crush phosphorous into a fine powder with light careful strokes (so it doesn't ignite). If ignition occurs nevertheless, add some water to the phosphorous before crushing it. Stir the phosphorous & a small amount of (preferably distilled) water into a light runny paste. Set the fired case vertically on the table and apply a few drops of the paste into the case. You have to make sure your solution is covering well above rim level, as the phosphorous' actual volume will be much smaller once water evaporates. Now let the cases fully dry for a few days. Then, find yourself a piece of plastic pipe (large solid straw, etc) that has a slightly smaller diameter than the mouth of the RF case you're working on. Insert the straw inside the case and use it to cut away the extra phosphorous from case walls, then scrape it away with a toothpick & recycle it for later use. Once the extra phosphorous is cut/scraped away from the bottom & case walls, you will end up with a case which only has the rim completely filled with phosphorous... with no missing spots!
Finally, add BP + an appropriate round ball or bullet on top and fire away!
(In theory, this should work very well... but be advised that I have never tried it though. The idea came to me last night when I was reflecting upon how this could be accomplished in an easy, cheap and effective way).