Time to become Dr. Frankenstein then, muuuhhaaaaaa......
Converting figures for everything is a stage you'll want to get to, but it's fairly basic modeling.. You can start simple by turning heads and raising/lowering arms, then get to mixing up legs, arms, and heads with various torsos... The key is to cut them at their natural joints and use a wire inserted into rilled holes to keep them in the positions you want.. then simply fill with putty and sand..
In Shep Paine's book, How to Build Dioramas, there's an excellent chapter on converting/painting figures and it's must-have for the modeling/diorama builder's library.. Frankly, if you only ever buy one modeling book, this one's the bible.. Must have..
I don't leave the hobby shop without at least a figure set or two.. Currently, I have about 300 figures (and some 50 or 60 figure sets I've not gotten into yet) on-hand.. Some sets I have multiple copies of, just because there's ONE figure i gotta have..
I keep them sorted by country, era, and scale, and also by legs, arms, heads, torsos... Nobody will say to me at a contest, "Oh, I see you used the Tamiya US Infantry set for the Sherman diorama.", lol... 'Course, some figures, like the 1/48 TBD pilot I used for "Uncle John" is a stock figure, but even 1/48 figures can be converted/mixed up... You also can cast copies, when you think you're ready..
I can't remember if I cast that TBD pilot, or he's an original, but he's likely a copy.. I think I cast about 12 of him, as well as copies of the Monogram F-86 pilot, the Me 262 pilot, and gobs of the paratroopers from the C-47 kit..
I'll write a primer one day on "casting/converting figures made easy", I think.. Be a big time-saver for sombody who doesn't want to follow my learning curve, lol.. I spent a lot of time (and money) learning how to do it cheap...