Not sure if you are a development house or not, but if you are, IBM has programs where you can lease(cheaply) a box for dev purposes. Contact the IBM Business Partner division for specs..
The snag in your request is “using the latest OS400 versions, without having to purchase an iSeries.” I don’t think I’ve ever heard of any system vendor having such a program. Best you can probably do is apply to a Business Partner or Independent Software Vendor program with IBM to become eligible for their 1% lease offers.
Otherwise, the best you’re likely to do is a time-share account with one of the few i5/OS sites on the internet or a purchase of a used system. I haven’t seen any timeshares offering current releases, and you’re unlikely to get a used system newer than a couple releases behind.