I had a piece of tiramisu that had a layer of mascarpone that really balanced the other flavors out perfectly. At the grocery store it's sold in such little/expensive containers I was looking for an alternative homemade version - any ideas?
Yes, you can. You'll require heavy cream, an acid (vinegar or tartaric acid), au bain marie technique and patience and a refridgerator.
Here are some fairly complete, although fairly long, instructions for making homemade Marscapone.
Any cream cheese (such as Philadelphia) could make a reasonable substitute, with extra cream added if necessary.