Holdfasts are a great idea! Forging sharp corners can be difficult, however, especially without an anvil. Depending on the style you are going for, you might be able to get away with a gentle 1/4 or 3/4 circle instead of a corner.
For an anvil, if you have a heavy sledge, that can work fine if you strap it firmly to a board, just don’t expect to be able to move a lot of metal very quickly. Otherwise, if there is a scrapyard nearby, look there for a block of steel. Mild steel will work fine, but if they have some sort of tool steel that would be great (4140 is excellent), but I’ve found that most scrapyards cannot/do not identify grade of steel. Or, if you have a few dollars to spare, check out Old World Anvils and search for their 4x4x4 cube anvil. It’s primitive, but awesome. For hammers, almost anything other than a framing hammer will be fine. I use purpose made forging hammers, but a cross peen or large ball peen will work great. If you get one from the hardware store, make sure it has a flat face (none of that textured ring business) and round over the corners before using it. Also, avoid fibreglass handles like the plague. A good old wooden handle is the best you can get.
Forging a holdfast is great, but be weary of how thick the stock you use is. I’ve made a few, and universally, the corner is the failing point. Make it as thick as possible. Without a solid corner, it simply doesn’t work. Mild steel from the hardware store will work if it is large enough. I would actually avoid high carbon steel for now because of the learning curve with working temperature range and the trouble of heat treating it later.