My first suggestion is the reformulated Worth Pour Homme. It's much more old-fashioned than the original, which is somewhat ironic. The base is more leather and soapiness than earthy/oakmoss, but the oakmoss is certainly present. It doesn't have the same green opening as Paco and the like; it's smoother and less sharp, and comparing the notes to Imperial leather shows a superficial simmilarity, but notes can be misleading. Still for $15 per 100mls (believe me it punches above its weight and doesn't smell cheap at all) it could be worth a look, for about the same as a sample of something vintage.
A more authentic, but costlier alternative (which seems much closer to Imperial Leather, structure-wise) might be Monsieur Worth Triple Cologne which is from '69, but it was old-fashioned even on release. Unlike other more popular classic fougeres there isn't a huge demand for the vintage and you can find it on Ebay for under $100 (100ml splash or spray) it shares lavendar, bergamot and lemon in the opening with Imperial Leather (though it also has Petitgrain, Eucalyptus and Rosemary) the middle is nearly identicle save the Worth offering has vetiver in place of patchouli and both have a base of musk, tonka, amber, oakmoss and vanilla. That said I've never smelled the Imperial Leather fragrance so I can't compare, but you could always look at the reviews for Monsieur and see how things pan out. For me it's a go-to classic, but then I'm very fond of the classic Worth. Many reviews cite an overwhelmingly dominant and very bitter oakmoss presence, but my bottle is more smooth at the base and I think this bitterness could be due to note degredation or some other product of ageing. Most of the bottles I've seen for sale contain very dark juice which is not the same shade as it should be, perhaps due to the splash-bottle presentation (I've mostly seen splash bottles around) as these can let much more air in.
+1 Mouchoir de Monsieur if you can find the vintage, but sample first as the civet presence turns many off.
+1 Wild Fern
Also check out the vintage English Fern by Penhaligon's if you can find it. This is - in my estimation - the definitive classic fougere, but the reformulation is quite tragic. Penhaligon's seem good for old-fasioned fougere's in general though, could be worth looking into their range.