Choose where to buy motorcycle parts (OEM, aftermarket, old stock or used): step by step

There are four main kinds of part. Each has good and bad points. Take the old part, the model and the year to the shop.

The four kinds

OEM (original equipment): made by or for the bike maker. It fits exactly and costs the most. Start here if price is not the problem.
NOS (new old stock): an original part that has been in storage. Steel parts keep well. Rubber parts age, so check seals, tyres and hoses of old stock.
Aftermarket: made by someone else. Common parts such as filters, pistons and valves are often as good as the original and cost less. Fit is usually fine, but check.
Used: taken from another bike. Often half the price. It is a good choice if the part is not worn out or if no new one exists.

Write down the make, model, year and engine number. Keep the old part to compare.
Look for the part from the maker first. Ask a local dealer.
If it is too dear, try aftermarket brands. Ask other owners of your model which work.
For old bikes, try old-stock sellers, online shops and scrapyards.
Check size and shape against the old part before you pay.
Ask about returns. A seller decides how a warranty is handled, and electrical parts often have no warranty.
Buy from the shop that gives you advice. It needs the sales to stay open.

Many imported bikes sell parts as complete assemblies (whole carburettor, clutch or magneto) which also fit other makes. Check the shape before you buy.
Piston and ring sets may come only as a set with the cylinder. Oversize pistons may not exist.
Ring sets may have no marks. Check Fit piston rings.
Manuals may not exist. Use general specifications from a similar engine and measure parts.
Find the same part number used by a major make if you can.