There's no such thing as a workaround I've ever heard of.
Just grab a retail copy of Leopard (must be the black disc, not the one of the gray ones that proliferate on ebay - they won't work on your machine at all). Probably the cheapest place to get one is to call Apple. They still have some and most people are selling their old ones at a premium.
The Leopard installer refuses to let you install it on any Mac slower than 867 MHz. If your mac fits this description then you have to use a work around to get it installed. And here is a nice link explaining everything you need to do to get 10.5 working on a low end Mac.
Others may disagree with me but if the OPs machine needs that workaround it may not be worth it. This is not to say anything against the workaround. I have never used it. It is just that I think Leopard would drag a bit on that vintage a machine so unless the goal is to get some of the specific Leopard features I might be inclined to stick with Tiger.