(I don't expect people to read this thread. I'm just putting my findings down, but feel free to comment if you like.)
I've been using the information on the Outlaw Star Wikia for a while to find these extra chapters. However, I have reason to believe they got it wrong. I decided to make this thread as a log to record progress in my acquisition of these Ultra Jump issues. Here's what we know so far, and what I already own.
ULTRA JUMP November 20th 1998: Outlaw Star #18 (Intermission)
ULTRA JUMP December 18th 1998: Outlaw Star #19 (Intermission II)
ULTRA JUMP March 20th 1999: Outlaw Star #20 (Sacred Ground)
ULTRA JUMP May 13th 1999: Outlaw Star #21 (Chaser)
I found Intermission included in one of Takehiko Ito's doujinshi. But getting the other three has proven no easy task, since these are issues that are almost two decades old. I think I've found the Ultra Jump issue that has the final chapter, Chaser. So, what's the problem? It will set me back over $50. I'd probably say closer to $70 or more with the costs of using a proxy and shipping. So I'd really be taking one for the team here. I could wait for a cheaper listing but I'm not sure when or if such a time will come. Now, about that misinformation, the Wikia says the chaser issue is from May the 20th. The information above came from Wikipedia (I think) and says May the 13th. But...
Clearly, you can see this issue is from the 25th of May, 1999. So I was wondering, maybe this is a special issue and Ultra Jump sometimes doesn't run once a month. So I consulted Yahoo Auctions' feature to search closed auctions...
While this user did not list the Chaser (or so I believe) issue you could see in the first picture, we can see a few things here. First of all I should say, the site that listed the Chaser issue showed it as issue #29. We can see the March issue that's supposed to have Sacred Ground inside is listed as #27 in the listings screenshot. #28 is April, and #31 is shown to be the July issue. You do the maths. Now I see there was no special issue and the issue dates reported by Wikipedia and Wikia must be incorrect. Therefore, I'm confident that by getting the 25th of May issue I would receive the Chaser chapter. But if anybody can advise otherwise, let me know.
Also, I think the Chaser issue that was listed at over $50 must be in pristine condition, which I don't care about since I'll be debinding the books to scan and touching up in an eventual scanlation. The four issues above were listed only a month ago for a few dollars each, and generally the 1998/1999 issues probably appear every couple of months, so I don't mind waiting a while to save money. I've tried using automated search e-mails before but they don't work very well, sadly, so I'll need to remember to check every day. (Perhaps I'll find a Gilliam II soundbite to play as an reminder.)
Anyway, that's the story so far. I've stalled scanlating the manga for a while now, and there's an extra project that has priority over it. You'll hear about this project in a month or so, or perhaps earlier as a teaser trailer. Or you can get early information on it by becoming a Nexus Crusader and gaining access to the Galactic Leyline. (As of now no exact rules for this—you'll be picked by becoming an enthusiastic and regular poster.)