Does anyone know a place that is selling individual monster groups from De1 expansions?
Monster miniatures for sale
Try Auggies games .
Doesn't look like Auggie's has any figures left, though. Assuming their inventory is up to date, anyway. And since 1E is well OOP, I wouldn't count on them getting more.
The best place to look is probably eBay and such. Assuming that prices are as high as ever, though, it may be cheaper for you in the long run to just buy the H&M Rehash packs that contain the groups you're interested in.
In that case, it doesn't look like he has ANY Descent in stock.