There are many sellers on eBay who are coin dealers, so finding a sixpence won't be a problem. You could even try asking for one with the bride's year of birth if she is over 40.
For the engraving, most malls have a shop called Things Engraved. It would be best to have the work done at one of those, so you can inspect the engraver's previous work. I would not advise having it done earlier.
Or ask at local jewellers . They often engrave rings , lockets and the like.
For a personalised garter, look here;
http://www.ebay.ca/sch/i.html?_from=R40&_trksid=p2050601.m570.l2632.R2.TR12.TRC2.A0.H0.Xblue+garter....
Start your Search with the Highest Price and drill down to your budget area. Do NOT allow an Asian seller to tell you it will take over 30 days for arrival.