A Shambler is a big white furry critter. If you haven't caught one yet, it's a crapshoot to get them to show up again. Dr. Alpha's island is a sort of safety valve since it has most if not all types of cryptids present, but people tend to waste their chances there and just shoot everything that moves.
You did get a live Sorcerer, right? Research the follow-up topic that this guy unlocked, otherwise getting a live Gillman Deep One is hard. But even then, it ups your chances from 10% to 28% per month to get a relevant mission. Not capturing a live Deep One when they were plentiful was a mistake.
Aliens will become more frequent in 1999, but the entire problem with that is you want Promo III to prepare for them, not the other way around. You may still manage, since ETs are probably the most guaranteed to show up among the four paths to Promo III, now that you've lost easy access to the others. And there are small, easy pickings among the UFOs.
The relevant stage of Cyberweb is probably too hard to battle through without Promo III.