I have had to go into emergency rooms with no insurance or money, and it may well be a method by which you can get a colonoscopy---however the doctor would have to deem this test to be necessary. If he/she does not, then you will have a costly bill on your back for nothing.
Hill Burton contracts with medical facilities around the US, offering and free and reduced medical tests and treatments based on income. It may require traveling out of your locality, but most states have a least a few facilities, if not more. Here is the link to its website:
Also you might check into any medical schools in your area. Some offer fee based care, so that students may learn in the course of a patients medical treatment. Generally, seasoned doctors oversee the internists in training.
And began a process in looking into getting maybe state medical assistance. Social workers may be able to direct you to places which may be able to help you get tested as well.
I hope and pray things turn in a good direction for you, and that all will be ok.