Thanks all for your comments!  I realized that the LC575 board bus is 33mhz, so I best use a 33 and avoid clock problems. After much searching I found a seller on ebay with reasonable prices.  He's got 25, 33 and 40 mhz chips.  I ended up ordering this one:
 

http://www.ebay.com/itm/371085061798?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I'll put it in with a 486 heat sink (hopefully fan wont be needed).  Hope it works!