I have one of the Holga lenses for Canon mount. It looks to be a standard Holga lens, but as you state it wouldn't vignette on the smaller film areas. They have added a metal strip with small circles running around it at the rear of the lens to give the vignette. It sort of works but also looks a bit fake.
If you are just after a meniscus lens try looking at those science education supply places - they sell a lot of stuff like that, some of it on ebay. If you are planning to put it in your own barrel they will describe the lens in terms of diameter and focal length so you should be able to be sure it will work as you expect it to.