There are a few papers out there on twitter spam - since spam is short lived, your best bet is to follow the data collection process as described in one of the papers. You could listen to the sample stream, manually annotating anything that looks like spam to build up a dataset. Popular trending hashtags often have a few spammy accounts posting for attention.
A more questionable way is to find one of those scammy “followers for sale” type offers and buy some accounts to get a set of spammy users quickly (but this approach could go very wrong getting your account associated with spam, or just you getting ripped off - not to mention all the terms of use violations etc.)