1. The use of the word 'digital' has really gotten out of hand. If you want a muslin backdrop, then ask for a muslin backdrop. No matter what some advertiser has said, muslin ain't digital.
2. The easiest possible way to relplace backgrounds is if your original shot is with your subject in front of a contrasting, solid color. That makes it very easy to isolate the subject. Think chroma-key.
3. Do you have a gray wall? A gray/putty/taupe computer case? Take a digital shot of those, even up close, and then use something like that for your background. Just because someone sells something labeled as 'digital' does not mean it is a requirement for you to buy it!
--
Crime Scene Photography