Good question, actually. Sheesh, if you can find them way back in an old drawer. There's some on Ebay, but not worth the price for your purpose.
Maybe you can still find them by film behind the counter at Walgreens etc., but doubt they're very common (or cheap). What works well, though, for cheap, are Xmas light bulbs that go on sale for pennies after the holidays, but are cheap anytime. There's plenty of tutorials available for making them, but it's probably cheaper, easier, and faster to make your own nichrome igniters. And the "reach" of either camera flash bulb or Xmas light filaments isn't too great. Probably going to run into difficulty getting them inside a commercial motor nozzle, and they're not very durable. Have potential uses, and I emphasize "potential", in fireworking, but that's a pretty ghetto approach with mediocre reliability... Nichrome is about $6 for a 100-foot roll in gauges 34-40, which is the balance point between adequate resistance and durability--smaller diameters are useful for bridgewires over chipboard pieces, but those, too end up too large for most nozzles (LPR/MPR). Larger diameters heat up fine and are often reusable (after cleaning off corrosive burn products and redipping) but require a hotter energy source (think motorcycle battery or LiPo with low internal resistance and good amp output) to heat up instantly, especially with clustered ignition. A good reference, with decent discussion of energy requirements vs resistance, is available here:
https://jacobsrocketry.com/aer/ignition_and_igniters.htm and at Nakka's EX site:
https://nakka-rocketry.net/igniter.html . I highly recommend you do not mention specific chems on this site. If your Ebay nichrome rusts, then you got ripped off--buy from a reputable vendor--there's plenty around. And remember, not all nichrome is identical--there's varying alloy ratios that affect resistance for a given gauge. Nichrome 60 and Nichrome 80 are probably the most common. There are easily located tables available that provide resistance per wire length so you can easily calculate igniter and total circuit resistance.
Oh, and here's a link to an
NAR webpage that details construction of large igniters for rocketry, believe that??? Ooooh, bet the NAR is on the ITAR hitlist for that, no? Sheesh. Anyways, they'd suck for small motors but easily modded...
https://www.nar.org/fai-spacemodeling/construction-techniques/igniters/
Last, if you're upgrading crappy Estes igniters, then do yourself a favor and carefully chip off the white crap they use in place of their old pyrogen--all it is is stankin' cornstarch and glue and it will impede your highly functional upgrade. 2.5 inches of cheap wire and a dab of cornstarch/glue for a buck? Bwaahhh hah ha ha ha!!!
Edited: Aha, I should have read more closely. Clicked on "Recent Posts" but didn't realize this thread was in the Recovery subforum. You're looking to ignite ejection charges, not motors. Same principles. Less worry about exact sizes. Simple to make a reliable igniter that'll fire off of a 9-volt battery. As always, test your igniters so you know well your all-fire current and power needs...
Attached is a PerfectFlite photo tutorial on making X-mas light ejection charge igniters. It's pretty straightforward...