What's your desired end result? Knowledge? Certification?
Most switches run the same software, so let your budget be your guide... Catalyst 3550/3650 are end of life, but run modern iOS... Could be a real decent starting point?
For CCNA, 2950s are dirt cheap at $100USD or less, but you're limited to 10/100 speeds only.
For CCNP, you'll want to look into a multi-layer switch -- 3550 series at least.
Be prepared to do a bunch of reading at Cisco.com to make sure you're ordering the right model as there's multiple sub-models for each main series.
@Nightfall GNS3 / Dynamips is good for routers, but because (Cisco) switches uses proprietary ASICs that can't be emulated by a computer CPU, that's not really an option here.